MY 2004 CHEVROLET VENTURE RADIO HAS BEEN ...

My 2004 Chevrolet Venture radio has been draining my battery. The mechanics discovered that the radio was drawing nearly 3 amps after the car is locked and off. They pulled the fuse and of course this solved the problem. Since the battery was over 5 years old I decided to replace the battery with an Interstate battery. Problem is with the radio fuse pulled the remote locks and power door will not function. I purchased a new s0ony radio from a locksmith company which installed it. This solved the problem for about a week. Then the battery was dead on Saturday I jumped the car and took it to another mechanic who kept it over the weekend. He found that the new radio was going on and off when the car was off and nothing running. The car again was drawing nearly 3 amps. He discovered that by disconnecting the radio completely that the amps went to zero where they belong. I believe that the problem lies with the security system or the wiring harness. Can you possible give me some help as to what to have him test?