Haynes manual says that the headlight door fuses (90-92 'birds) are in UEC, but I could not find it. Could anybody describe where should I look?

Don't believe everything you see in Haynes.

1. There is no underhood electrical center. Even on the '91.
2. There is no fuse for the headlamp door module. It uses 3 fusible links for power. Check the junction block by the pass side headlight or whatever side yours is on, and down by the starter.

What did you mean as junction block? I see 3 fusible links in the wires that go from battery to somewhere under the engine (I think to the starter, but haven't checked it out yet). Should I check these?

The ones on the starter are a good place to start. IIRC, the main power wire for the module comes from there. If you haven't molested/upgraded your electrical system (the big 3 wires), the wire running from the BAT terminal on the alternator should run into the junction block (or the little wire from the positive battery cable, either/or).

In my 91 TA I have 2 fuses labeled HDLP 1 and HDLP 2 If I remember correctly they are 15 Amp. They are all by them selfs with a flasher. if you look even with the left side of the console, behind the dash , just below the right side of the guage cluster. There is also a blank spot for where the rear defrost relay/module would be but I dont have that one plugged in. The only other way I can describe how to see it is lay on your back with your head by the gas pedal and look up towards the center air ducts.
its not a fun position.

I found these fuses and both were fine. So I disassembled the Headlight door module and guess what - there were two fusible links inside and one was blown. I replaced them with fuse holders and fuses and now both doors work fine again.