Audio Systems: head unit power problem, default beep, ford wiring

Question
I have a 2005 Ford F150.  I just replaced the factory head unit with a Pioneer DEH-P4800MP unit.  Everything worked great until I took it for a test drive.  When my truck starts moving, the head unit shuts off.  I believe I'm actually losing power to it because it beeps 5 times when it shuts off (this is the default beep that reminds you to remove your face plate when you shut the vehicle off).  When I stop at a red light, it turns back on.  It has nothing to do with what gear I'm in (reverse, neutral, drive) because it works in all of them as long as I'm not moving.  It also has nothing to do with the brakes because if I'm at drive through or something where I'm just coasting, it works fine.  I've even revved the engine in neutral to see if that shuts it off, but it still works.  It seems to shut off around 5 mph.

A friend to told me that newer fords adjust the power to the head unit based on your speed in order to make up for road noise.  Is this true?  If so, how do I get around it?

Thanks,
Scott

Answer
This is due to the difference in Ford wiring and ISO DIN wiring on all modern radios and the problem lies in the Accessory or +15 wire (RED) and not the Poss
(+30 yellow)
You should have used an adaptor harness which usually cures this.
Failing that, try taking a new Accessory feed from the fused side of the fuse box such as ciggy lighter (only on when ignition is on),
Peter