Nissan Repair: 1986 300 ZX electrical, radio antenna, universal joints

Question
I helped my boyfriend purchase a perfect body vehicle for $200, but it has electrical issues.  The previous owner says a squirrel ate through the wires, but I have taken it to a mechanic and most of the issues have been resolved.  Final problems are the radio antenna does not go up, so the radio will not work.  Once in a blue moon the antenna will decide to go up and the radio is great.  Another issue is that the heater finally works, but the air conditioner dies not.  Finally, when you put the car in reverse it has begun to make a popping sound like something is broken.  

When you respond, could you please answer as if you are explaining it to a 5 year old blonde girl, as my boyfriend expects me to show him what is wrong.  I would greatly appreciate any help you can give me.  

Answer
Marilyn,

The radio antenna problem is probably the antenna relay needing replacement.  If that is not the cause remove the radio and check for the signal to the antenna to see if the head unit is sending the signal to get the antenna up.

The airconditioner probably needs charging.  Have the system pressure checked.

Check, the universal joints ont eh drive shaft and the engine mounts for the reverse problem.  Otherwise, it is probably the reverse band.