Ooo thats a toughie, here is how i have done it in the past.

First of all you will need to get the plug connector for the stereo you want to install

(don't know if you already have this but you will need to get one).

Next you get yourself an MR2 to ISO adapter which will just be a connector that will convert your standard fitment to iso

(so that most stereo's would be able to plug right in).

Next what you would do is cut the wires on the iso adapter and use connectors to pair them up to the ones from your new stereo plug.

It may help to have a voltmeter and wiring diagram for the wires to be careful of the all important battery live, ignition live and earth.

I hope this makes some sense.

Cheers

Scott

=oP