monty400k you are meant to bypass the amp in your case, which I believe the Extension cable does. It's been mentioned before, but I'll have to dig up the info.
I didn't realise this but I always saw me having my car with no sat-nav as being a hinderance. In some ways it wasn't. I hope you find an easy way, as my next rather larger update does include some info about the HU .
Yep, that makes sense, been doing some hunting around online and it's a common issue, just need to find a definitive way to do it!