Low ignition voltage

[Santa79]

I've just come to write in my new stereo to the van. I thought I'd look into why the previous owner had wired it in permanently to the starter battery. 
So I wired the new one in correctly to a switched 12v and permanent... Queue the stereo coming on for half a second then going off.. 

Turns out the switched 12v is in fact only 10.5v at the back of the relay panel and into the (previous owner) fitted multi cigarette lighter block stuck to the dash.

​​​​​​I've had a look at the earth points and they are in good nick, but I'll give them all a good clean up whilst the glove box is out anyway. But I was wondering if anyone might know of any common issues that could cause this, before I go chasing voltage drops around the place!

Thanks 
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[robertneill602]

As far as I know there's only a single main feed wire from the battery or starter solenoid 30 terminal. Overtime this can become somewhat inadequate and it was a known problem on older VW's. Run other cable alongside the first connecting to the terminals designated H/30 on the fusebox/relay plate. On pre 84 models using the 12 way Continental fuses the connection is to fuses (7),8,9 busbar. Fuse 7 is often joined to 8 and 9 by a 4mm squared wire link. Would upgrade this to 6mm squared and thus form a ring connection with the new incoming link wire/cable from the battery/starter solenoid positive terminal.