Just bought 12 volt fridge on e bay

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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by fourfourto:
I guess the right socket is 30 amps because it also runs the cargo one.

Is there a way to tap into the dome light it has rundown protection after 20 minutes.Its better for the fridge than rap because rap goes of when you open and shut door if hooked up to dome circuit it would stay on for 20 minutes after leaving truck. </div></BLOCKQUOTE>

Yes, all interior lights have rundown protection. (Which I'll add here for others, rundown protection and RAP are different.)
However, the draw of a the cooler might be too large for tapping into the dome light circuit, unless you wire in a relay. Just wire in the coil side of the relay to the dome lamp, and run a B+ line (could be one from the accessory plug) for the switched side, that would work and not overpower the dome fuse or more importantly the dome wiring.

Using your idea, it would not be hard to make the rear accessory plug (or one of the front ones) turn off after twenty minutes, just be adding in a simple relay somewhere in the rear panel.