Radio woes

[ironmikea]

Greetings,
I have a 2004 boxster and am having a radio issue similar to what I saw in other posts. When I purchased the car the radio wasn't working, the dreaded PROTECT mode so I decided to replaced the radio with a new Alpine radio. I ohmed out the wires since the colors described in the project 101 book were different and located the switched 12V (blue-white), constant 12V (red), ground (brown), etc. The radio install was simple using the aftermarket female harness plug so all I had to do is connect this harness to the Alpine radio harness. I tested out the radio and it seemed to work fine. I proceeded to drive the car later and noticed when the car was moving the radio would shut off. I removed the radio and placed a volt meter on the switched 12v wire and ground and noticed the issue is the switched 12V power is initially ~ 12V when the key is turned to accessories, approximately 14V when the car is running but once I see the speedometer digits change from 0, the 12V switched power drops down to approximately 5.5V. When the car comes to a stop (0 on the speedometer) the 12V switched power jumps back up to approximately 14V. The constant 12V wired stays constant though. What appears to be happening is the gauges are causing a voltage drop which doesn't make sense I would think that all the accessories are installed in parallel with the voltage source. Any assistance in troubleshooting this would be greatly appreciated.
Thanks in advance.

[mikefocke]

Might it be that that switching signals the radio (or NAV) that the car is moving and thus can't be changed until you stop? Many work like that. So you need a constant positive wire even if it has to be from the fuse box?

[ironmikea]

Thanks for your response. Interesting point that I didn't think of. It's consistent so maybe it's not a short. What I will probably do is what you suggested, which is create a 12V source.