It depends on how dim, it is as simple as you may be taking more than the alternator can supply at the idle speed.
Owing to the age of the car though, it would be a very good idea to clean all the earths from battery to body and engine to body, mk2 lights are not the best, bring them into the 21st century with a wire renew and Osram Nightbreaker bulbs.
All wire builds up resistance as it ages, so with things like lights you would get a good improvement normally from using seperate relays to the headlights, but anywhere you see a earth clean it.
Check also the charge rate with it idling and nothing on apart from the engine it needs to be at least 12.8 volts, with everything on it will likely dip below this but when raised to 2000rpm or so it should rise above 13.2v or so.
tabetha