The MR2 uses external amplifiers and sub-woofer amplifiers and so the installation of a new head unit needs a little more thought than for most cars.

You need to provide a little more info.

What is the year and model of your car.

Do you just want to swap the factory unit for a single cd-receiver or multi-cd changer etc.

etc.

Does your new unit have built in amplifier and/or outputs for external amp.

etc.

etc.

You need to decide exaclty what you want to install and then you can get accurate advice on what you need to do.
