98 Explorer Sport Wiring Issues

I have a 1998 Explorer Sport, and I recently had a amp/subwoofer installed. The problem is that after it was installed I've had problems with my alarm system. Previously, I could disengage the alarm by using my Remote Entry unlock, but after the amp was installed it doesn't work. I have to unlock the suv, and after getting in I can hold down the unlock button (on the door) and it eventually gives me the (beep) sound and my car can now start up. Without doing this the car will not start. I am pretty sure that the alarm is aftermarket but I bought the truck used and it was already installed. A friend of mine told me something about there being more than one power supply to hook the amp to, and having them on the same supply may be interfering with the alarm power. I need help on finding this auxillery supply or if anyone knows of another way to tackle this problem I would be glad to hear about it. Thanks.