Your post doesn't say if your reception has always been poor or started acting up. I'll explain how it works before I offfer up any suggestions. . .
Your FM antenna (also known as a Diversity Antenna) consists of two seperate antennas (elements) bonded to the ground plane on the underside of the rear decklid. The antennas are made of 20 gage, twisted pair stranded wire. A left and right antenna module condition the AM and FM signals. The primary antenna, bonded to the right side of the antenna plane receives both AM & FM signals and routes them to the right side module. The left side module receives only FM signals.
When the radio is energized, it sends an antenna enable signal (power) to the right module. When the AM or FM band is selected on the radio, the right module routes the antenna signals (from the left and right elements) to the radio.
When the FM band is selected, the FM composite signal is sent to the antenna module. Modulated on this signal is the strength of the signal going to the radio. The antenna module uses this signal to switch back and forth to each antenna element, mixing the received inputs to create a stronger signal to send back to the radio. A dual-channel o-scope could be used to measure the phase and amplitude of the FM signals as outputs of each element. You may have a faulty antenna module, which would explain why the signal is fading in and out.
A Tech 2 can check it out and identify any faulty components.
If everything checks out, there is a replacement antenna available from GM that looks like the rear-mounted "Wart" with a 6" stick on it. The kit comes with a template for drilling through the rear decklid (ouch!) and installing the antenna, plus a metalized foil stick-on antenna for the area normally covered by the rat fur on the underside of the decklid. I bought one of the kits when I wanted XM, but couldn't bear to bore holes in my car and break the rear deck's clean lines with a protuberance. I ended up installing a low-profile XM antenna wth a magnetic base on my dash instead with great effect.
Thanks for the info. I have taken the car in and they said they could not find a problem. It has always done it. It seems like its on the edge and even the weather seems to effect the reception ,the st light seems to go on and off with the reception. I don't know if its just his city or what my old 1987 supra had issues as well. I do know my other car and truck do not have issues. I don't like the idea of those wart units but I can't stand poor working equipment either. I have herd of FM signal boosters but I don't know how well they work.
Because of the way the XLR's diversity antenna works, I don't know if an FM amplifier would be of any help. The foil backing in the GM kit might offer some improvements by increasing the area of the receiving plane that the twisted pair wires interact with.
I don't know what specific tests your dealership ran. For troubleshooting purposes, if no diagnostic trouble codes are flagged by a Tech 2, the dealership can test receiver sensitivity by tuning to an FM station and measuring 4-5 volts from the FM composite signal to ground on the radio antenna module. The other paths are connector integrity, antenna module or radio replacement. It could be something as simple as a poorly-built connector that has a path of a few strands of conductor; or something similar with a ground path. I've fixed plenty of (hidden) workmanship problems.
Mine works great, rain or shine, city or country; --yours should too. Too bad you aren't closer, it could be an easy, two-beer fix.
The power-on reset you're doing is re-initializing a circuit; this points more to a radio or antenna module that is faulty --could be as simple as a bad capacitor. It's tough to accurately xshoot a problem like this without being there. A module swap would likely isolate the problem quickly, but many dealerships are loathe to do that. If you can show them a repeatable problem, you're more likely to get it repaired properly. Like I mentioned before, the design is sound, and your FM receiver should work as long as you're getting a channel lock. You could remove the rat-fur liner from the underside of the trunk lid and manually disconnect then re-connect the antenna module to interupt/restore power and see if that changes anything. If it restores the reception, I would lean more towards the antenna module as being a prime suspect. Still, it's a good idea to check connector integrity, ground paths, etc as well. The key to good troubleshooting is understanding how something is supposed to work and then proving that it does. Whatever is left is suspect until proven otherwise.
--If you decide to remove the liner, stock up on extra plastic button locks. They break easily during removal, but luckily are cheap. I keep a dozen handy.
--One issue that's common to the XLR, (and a lot of other vehicles as well) is receiver gain between XM and Sirius channels. While I get good reception with both, the Sirius channels are appreciably louder (so much for Automatic Gain Control.)
I finally have an update on this issue. The dealer has been in touch with GM tech support and they have advised to update the program on the radio however there is no update available. They are replacing the whole system under warranty. The only issue now is the radios are back ordered.
GM has a replacement antenna kit (looks like the wart, only it has a stick coming out of it at an angle) which comes with a large stick-on ground plane that adheres to the bottom of the rear decklid under the liner. I bought it a few years ago to add XM until I figured out a way to install a tiny, dash-mounted antenna and not have to drill into my rear decklid. Anyway, installing the ground plane may increase your receiver sensitivity.
I just retrieved the box from storage. The P/N is: 25821144. The part name is: XLR Antenna Service Kit. After reading through the instructions, the foil is used for the (new) deck-mounted antenna's ground plane. (Beneath the rat fur.) The six-inch mast that screws into the wart should offer better FM reception. Let me know if your dealer thinks this is a worthwhile upgrade and I'll ship it to you. It's just taking up space here and is of no use to me. Maybe it will help you out.
I have the same fm reception problem and the dealer can't repeat the problem. If I switch from Cd to fm and back the problem is fixed. I have an 06 with extended warranty and only a few months left. How can I get the radio replaced they already replaced the antenna modules?
Take the tech for a ride and repeat the problem for them if necessary. Then it's their call how to fix it.
I just took my car in for a really weird issue that I couldn't fix and the dealer couldn't repeat. I explained how I could make the problem occur during normal driving and once they took it out for a drive, were able to figure out why it was happening.
It's all about communication. Dealerships really want you to be happy with their service --many dealership Service Managers get special perks for high customer satisfaction ratings (like trips to Hawaii, etc.) If the Service Advisor won't listen, talk to his boss, the Service Manager.