How would I go about wiring a 12v plug socket so that it only receives power when the car is on.

I'm installing some accessories and I would like one of my aftermarket cigarette sockets to only be on just like it is stock where you'd have to turn the key.
I would like to figure this out somehow with a relay I suppose to a bussman rtmr.
(Basically I'm wondering what to tap for the on signal like a switch and I'm also new to relays so any and all explanation would be great.)

 

Splice the wire to the current lighter circuit unless your planning to run massive current loads off both sockets at the same time.

That is what I would do no reason to get all fancy unless you want to .

Just what sort of load are you planning ??fuse the new circuit to that amount .

Be aware some after market sockets are rather poor quality .

 

For the "trigger" wire, use one of these,

ABN Mini Fuse Tap Holder & 15A AMP Blade Adapter - 2-Pack, 12V Add a Circuit Kit

https://smile.amazon.com/gp/product/B01BU59K90/ref=ppx_yo_dt_b_asin_title_o08_s00?ie=UTF8&psc=1

Find a circuit that is hot when the ignition is on and piggyback it onto that fuse. I don't know exactly what is ignition wired in the under dash fuse box, but I'm sure there are more than one.

Wire it with a relay as shown here,

https://smile.amazon.com/gp/product/B01BU59K90/ref=ppx_yo_dt_b_asin_title_o08_s00?ie=UTF8&psc=1

instead of "pump" you'll hook that to the hot side of the aftermarket cigarette lighter. Ground the other side of the cigarette lighter jack and you'll be good to go.

I know, we don't call them cigarette lighter outlets, but that's how I learned them way back then.

As Wyoming09 says, spend a bit extra on good quality sockets.