No power to ACT car video

We have a 2003 Town and Country, and have had three professionally installed ACT 8.4" car video players (ACTM84FDDVD) that have died. Each has been installed where the dome light would be. Each has stopped working. At first the DVD player works fine. Then the DVD has no power when the car is turned on, but after a time the power comes on while we're driving. Eventually the DVD will not turn on at all and it's dead. The second DVD player had a period where after if would finally turn on, it would loose signal. There would be sound, but no picture. It eventually died all the way too. The shop who is installing the DVD players says they're hooking them in to the radio. Our radio works fine. We have had a load test done on the car, and it is fine, too. Now we have the fourth DVD player installed, but we're afraid to turn it on until we know what's happening. Any ideas? Thanks in advance.