they have the years wrong here for the body styles. my 90 jetta is doing the same thing. same car as your 91. there is a sensor int he coolant bottle that shows if your low on coolant. teh wireing for that has a nasty habit of corroding due to how the sensor is setup in the bottle itself. trust me. i have been wracking my brain over this for 2 years now. is the guage reading normal? are teh fans comming on??
if its working right and the warning light is the only thing comming on then its a wireing fault and thats about it. i have temporarily delt with mine by clipping a lead off the LED inside the cluster. my car does not overheat according to the gauge, fans kick on and i have not lost any coolant int he last 30k with it this way. you should be fine.
as for the dash lights they may have burnt out or the switch has gone bad. changeing them is fairly simple once you get the cluster out. my suggestion is get teh GOOD repair manual on it. its by Robert Bently Publishing 1985-1992 golf \jetta reapir manual. includes 8 valve, 16 valve and deisel engines. that will take you step by step on how to do almost any repairs on your jetta. here is a link to the book at teh publishers website.
http://www.bentleypublishers.com/product.htm?code=vg92 hope this helps.