my bros mk2 fiesta lights go dim when most electrical things are put on we upgraded the alternator and the still go dim when most things are put on the battery starts the car normaly so we dont think that is the problem any ideas what the problem could be
blunty

 

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

 

the lights go really dim so dose the interior ones the stereo will turn is self on and off with the lack of power when most electrical things are put on like the window wipers and heaters

 

That's defo way to low, check the charge rate first, check at the alternator and the battery.
It sounds as though either the new alt is duff, or you have a major high resistance between alt and battery or battery and fusebox for so much stuff to be affected, batteries can suck a huge amount of power when duff, but you don't mention any starting probs so assume this is not the case.
If necessary temporarily run a seperate new heavy wire from the alt B+ terminal to the battery.
tabetha

 

well the only problem we have starting it is when the car has been running and every thing has been on ones its been turned off the car wouldnt restart as the battery was dead

 

Check your charge rates, a duff alt or battery most likely.
tabetha