help me

i have an 87 moght max and ther is something wrong i have no clue. a couple of months ago i changed the battery and alternator and now my battery isn't charging and i had the alternator tested and it was fine. well i can jump start my car and it will run but uf the lights are on they are dim and the cra runs funny if there off it runs fine but if i shut it off it won't restart. there is also a wire going down toward the alternator that looked like it came off of it i check the plug and other things and can't seem to figure out where it goes if anyone could help me i would appreciate it thanks


Brhalt