Which is a good fuse position to use for hard-wiring a radar detector . Want one that does not turn on until key is in Accessory position.

 

might have to get a test light and go to work, historically the radio fuse is typically a key on power source

also have you considered looking at I think it's wire tap - they have kits that tap into (prongs that insert) the power from the rear view mirror systems so you just have a short chord to from mirror to radar

 

Exactly what I did. Tapped into the wiring for the mirror and it’s switched power.

Radar Mount Mirror Mount Bracket + Mirror Wire Power Cord for Escort Max Max2 (3001102)

 

OP, I have an extra one if you want it you can have it. Amazon sent me two of them. PM me your address and I'll drop it in the mail.

 

I just did this not too long ago. Tap into the mirror. Looks and works great!!
2021 Ford F-150 Mirror Mount Radar Detector Install

 

Are there any dash cams that can be installed in a similar manner does anybody know? (Apologies if this is ‘hijacking’ the thread - no offense taken if someone tells me to move out!)

 

well. It’s a 12v connection so I don’t see why you couldn’t wire a dash cam into the mirror connection the same way the radar connector is connected. Not sure I would do both pull too much power and you may blow a fuse. Not sure how much that would take. Depends on the amount of amps each device is pulling.