Strange electrical problem. low voltage

I've had an ongoing problem with my 1988 clipper for about 2 years now.

started off that my car wouldnt turn over when it is warm, i put it down to the starter motor and eventually replaced it.

Fault has now come back so I have spent a little time trying to investigate.

after having a look with a multimeter, what is happening is, on occasion when the car has been driven when I try to restart it, the car doesnt turn over, when I crank I am only getting 6.8v to the "trigger" wire at the starter, i have traced this wire back and looking at the ignition switch, I only have 6.8v here too on both sides of the switch, I've looked in the fuse box and everything which is live when the ignition is off is getting 12v, i'm also getting 12v on the 2 thick red wires, i'm guessing one is from the battery and the other is the feed to the ignition switch.

before i rip my dash out tracing the wire, is there anything between the fuse box and the ignition or does the wire run directly to it?

has anyone else come accross this problem?
 

the red wire from the ignition switch should run straight to one of the red wires you have found on the back, so you are on the right track tracing them

before i pull my dash, i cant think of any obvious reason why i would have 12v on these red wires but only 6v at the switch, and i only get this when the cars been running for a while..

does the wire run close to the heater matrix? worst case I'll just replace the wire, but it seems a strange fault to me.

i would start art the switch end and trace it as best you can with the dash lower trim removed, the red wire runs in with the main dash loom all along the bulkhead then down to the fusebox.

all i can think of is its had some dodgy splices or repairs in the past, but usually if you mess with the wiring to say install an alarm you wouldnt touch the red wire it would be the black ignition or red/black starter wires you mess with