Check fuses under your dash.
I just had a very similar problem with my 2002 Tacoma. I added a super simple plug in trailer wiring harness the other day and then towed a trailer around for a day. I found out a day later that all my dash lights were dead. All of my turn signals, running lights and brake lights worked just fine. I figured if I screwed something up it would be with the turn signals or brake lights (since that is what I connected the new wiring harness too) but for some reason my dash was affected. I checked the fuse panel inside the truck and all the fuses were OK (including the fuse for "gauges").
I then did a search on a different Tacoma forum and found out that the fuse for the dash lights is in the engine compartment and not in the fuse panel inside the truck.
When you checked the fuses in the engine compartment did you check the fuse for "tail" ?. (I know .. sounds nutty since I'm betting your tail lights work just fine) Open up the fuse box and look for a fuse spot labeled "tail". See if the 10 Amp fuse for "tail" is blown. Replace that fuse and see if that solves your problem.
My guess is that the extra current draw required by trailer lights is more than the 10 amp fuse can handle. I replaced my 10 amp "tail" fuse with a 15 amp fuse. I have not hooked up the trailer since I replaced the fuse but an extra 5 amps should do the trick.
