Colorado 12v outlet

Cigarette plugs were never designed for constant relatively high current flows like a fridge requires and the standard wiring in a vehicle is rarely of sufficient size to run a fridge either.
Far better to install dedicated wiring direct from the battery to the required accessories and use proper good quality plugs and sockets like the Merrit/Hella or Anderson types.
With late model vehicles where the vehicles electronics control the alternator output they can often not supply enough voltage/current to charge batteries and run accessories.

I don't know the specifics about the wiring for the 3 acc sockets, but most vehicles with two sockets have them on the same circuit sharing the one 10A fuse.

If your ACC fuse is intact, I'd suspect the socket.
Some OEM cig sockets have a thermal fuse built into the socket. So that if the socket gets hot from high current (and poor contacts), the thermal fuse will melt and render the socket useless. Its designed to prevent fire.

What sort of fridge? 3-way fridges will always draw too much power for a cig socket. A modern engel will only draw 2.8A when cycling, so is usually OK. A Danfoss compressor fridge will pull at least 5A when cycling, so is more likely to cause a problem like yours.

Like Peter says above, get a higher power socket fitted, or an inline plug, such as an Anderson plug.

Hello col, I know very little about electrics but visited a Auto elec and they sought my problem of running a deep cycle battery in the caravan.They installed a dedicated heavy duty wiring from the car to the dc battery and problem solved see an expert!,

I own a 2000 Jackaroo & I do not have problem running a 40ltr Engel fridge from the the rear 12v plug. From past experience all car/4wd makers change things around from one model to the next model. If you are to remove the power outlet attachment & check the rear the attachment it may have a thin copper wire & by now maybe in 2 pieces. If this the case you will have to purchase another one that has a thicker copper plate at the rear. It does not matter what size they are this copper wire or plate is your fuse. Some car manufacturers only put in a thin copper wire and it is not heavier enough to charge a mobile phone.
If you are not sure seek the advice from a auto electrician.

Check out the Cigar and Acc fuse C12/20A

Cheers
Leigh

Yet again,

Is there any thing else in the car that's not working that may be associated with the power outlet circuit?

Update - thanks everyone for your help and input.

It seems the 12v acc system runs two fuses, one for the left front socket and one for the right front and rear socket.

Found one fuse blown, replaced and all ok.

Next job is to install 2nd battery, prob under the tub as there is not much room under the bonnet. Has anyone done this type of install and maybe offer some advice or tips?

Thanks again