radio- switched live problem?

hello all.
i seem to have developed an issue with my radio switched live feed? its only showing 6.5 volts on the switched live input, for some reason, and hence the radio will not work?
i have had bad earths before in the past, with the earth point in the radio well there. usually just clean it up, and screw it back in.did that again assuming it would cure the radio, but not this time, so i started investigating elsewhere.
i am running an aftermarket pioneer bluetooth unit, which has worked well for years.

i have a test probe now, and it shows a good earth on the earth point, so ruled that out.

so i have been looking at the wiring diagram (for 1990 model) and there does not appear to be a relay involved, otherwise that would be my first port of call.

so there are tow fuses involved A3 and B2 (both fuses are fine in there).
one thing that puzzles me is that when i put the test probe on each contacts on fuse holder A3, it shows earth for both points! one should be positive, right? does that mean its leaking to earth somehow??

am i missing something obvious?

oh forgot to say, the permanent live feed is there, so its just the switched live that appears to be iffy.

no ideas anyone?

it will just be poor contact or wire , just trace switched live back to 12v then replace from there

yes, thats where it gets tricky, as it disappears under the dash. was hoping not to have to remove the dash. could i wire a new connect from the back of the fusebox? never had one out on this car, so no idea what it looks like behind?

yes any switched live would do or even permanent if you so wished,switched lives are white

yes, thats what i was thinking. it will be more straight forward to make a new power supply.