Audio Systems: Car Audio question, jeep antenna

Question
Good Morning

I purchased a used jeep with an aftermarket radio previously installed.

Radio is an Alpine Model CDA-9856

I've had this problem since I bought the jeep.  The FM radio stations are weak, fade in and out and I only get one local station.  People have suggested checking the antenna connection at the radio. Checked it out and it appears to be connected tight.  Others had said that this is a common problem with jeeps and aftermarket radios.  Any thoughts or suggestions that I might try to use to troubleshoot.  thanks in advance.

Answer
Number one problem in that vehicle  is the ac condensation behind the radio, that ruins any radio factory or your alpine unit  best way to check your radio is to find or purchase a $10.00 car antenna and plug it into the radio  if you get the same result it will be a bad radio
if it works just find the problem is in the vehicle antenna
since i don't know what year it is there is no much i can tell you
good luck