Check your interior lights fuse. Is located in the engine compartment fuse cluster (driver's side under the fuse door).

 

I checked fusses this morning one cigarette lighter fuse was blown, put new one in still dont work. Fuse is still good.
I checked the 50 fuse underneath that cover was good. Checked the power block relay has 4 wires the orange one is broke. Corroded.

 

Can you post a picture with what you found (the "power block relay") I don't understand what you mean. Do you have power to the interior lights fuse?
Underneath that cover on the top outer corner there is a fairly large relay mounted with the connections down. The wires connected to this relay are fairly thick (~12 AWG) but one wire is significantly thiner (~18 AWG). That is the wire that provides power to the interior lights fuse. Is that wire OK (should be double ferruled with the large wire underneath the relay terminals).

 

Hey I got a 98 eldorado that was having the same problem. I looked at that relay and the wires broke in my hand they were soo corroded! I fixed the problem and all the lights work again.

 

You have a leak in there, water gets underneath the hood and run along the fender (on the strut tower) and flows down to the relays underneath the cover.

 

My 97 STS has the same problem - no interior dome lights, no glove box light, and no trunk light. Heck, the air does not work either. I checked the fuse & relays look good. Is it necessary to remove the plastic shroud to gain access the wires that connect to the fuse box?

 

I am having the SAME EXACT PROBLEM! I have no A/C (system is 2.5 years old), no interior lights (gauge cluster, radio, shifter, HVAC buttons) the only thing that is backlit is the headlamp control switch its self. Ive checked all the fuses in the front and back boxes. I relived that someone else is having this problem, and wondering if youve ever figured this out? Best of luck!

 

exact same issue as you guys, no lights, no ac - was a solution ever found?

thanks, Brion