'99 DeVille Electrical Problems w/Aftermarket Stereo & Fuse Block Rebuild

I have a '99 DeVille which I bought in November 2008. Mechanically it's in excellent shape, but it has had persistent electrical problems which, though they don't affect my ability to use the car at all, are still annoying. I will try to keep this as brief and clear as possible, thanks very much to anyone who has some input on this.

Ever since I bought the car, it has had an issue with the alarm system. Namely, once in a while (I drive a lot and it happens roughly once a day or every other day, with no pattern that I can identify except that it never happens twice in a row) the car seems to "forget" that it was locked with the remote, forcing you to open it with the key. After unlocking it with the key, you turn the ignition on and the alarm goes off. I know how to shut the alarm up with the unlock button after that, so I have just been putting up with the problem. After this happens, the Driver Information Display is reset to the default blank, and the trip counter is reset to 0 (I always keep it set on Engine RPMs otherwise, so I can recognize when it happens.)

This has happened a few times when the car wasn't locked, basically all the same stuff happens except the alarm doesn't sound. I noticed one time after it happened and I only let the engine run for about 30 seconds after shutting the alarm up, the buttons inside the car that open the trunk and gas cap door did not respond and the display acted kind of funny - The [P R N (D) 3 2 1] indicators lit up, but nothing else.

Now, the thing that complicates this is the car has an aftermarket stereo. (Alpine, professionally installed by a place that the dealership recommended) I also do not know if the alarm problem ever happened before the stereo, since it was installed the day after I got the car. The installer used a "GM Data Bus II Adapter" model GMRC-03 which I have read about, apparently this does *not* take care of the data passing through the stereo and you are supposed to install the stock radio in the trunk to preserve this. I have not had this done yet, but the problem still seems kind of odd because of how inconsistent it is and the fact that the keyless entry usually works fine. And naturally, no dealership will even try to diagnose it as soon as they see an aftermarket radio in there.

The second electrical problem started recently. The engine blew a head gasket and overheated (Shocking!) and while it was in the shop, the dealership told me that the fuse block under the hood was also extremely corroded - They said that the previous owner had even brought the car in with the headlights turning off randomly and the interior lights not working and had the same problem diagnosed, but not fixed. So I had them clean and rebuild the fuse block, and they replaced most of the relays.

Since the fuse block was rebuilt, the interior lights in the car (Dome light, reading lights, and the lights on the doors) no longer work, though they worked before this. The front and presumably rear cigarette lighters stopped working as well. I checked the fuses themselves and they weren't broken, and had the dealership look at it again without finding anything. Unfortunately I had to go back to school so said dealership is 300 miles away now.

Here's all the codes I pulled out of the computer:

P1520 History - Transmission Range Switch Circuit
P1527 History - Transmission Range to Pressure Switch Correlation

B1552 History - Keep Alive Memory (KAM) Error

U1255 History - Class 2 Communication Malfunction (Serial Data Line Malfunction)

C1232 History - Left Front Wheel Speed Circuit Open or Shorted
C1234 History - Left Rear Wheel Speed Circuit Open or Shorted
C1235 History - Right Rear Wheel Speed Circuit Open or Shorted

B1971 History - Inadvertent Power
B2471 CURRENT - Interior Lamp Fault

Of the History codes, B1552, B1971 and U1255 stick out to me as possibly relevant to the alarm issue. Obviously the only CURRENT code, B2471, is relating to the interior lamp though it doesn't tell me a whole lot.

Any thoughts on this whole thing? Should I probably bring it to an installer with the original radio and ask them to mount it in the trunk?