I have a 91 sidekick. Ive been thinking about putting in an automatic. I have a 3 speed auto that I purchased a while back. How hard is it to convert to a automatic from a manual? What do you have to buy?

3spd Auto trans, TB kickdown w/cable and the shifter w/cable. Some will say you need the wiring and ECU to get convertor lock but that can be done with a switch and handled manually.

will a automatic transmission bolt in without any modification?

You might want to get the diffs from the Auto too or it will be very low geared.

Apart from the ECU wiring and stuff, I think you will need to change the Diff shafts as well.

Apart from the ECU wiring and stuff, I think you will need to change the Diff shafts as well.

Absolutely not with the 3spd, it is the same length as the 5spd and use the same driveshafts.

5.12 gears, a 3spd and 31" tires is just about perfect as far as gearing goes.

There is no need for the auto ECU and wiring harness to be swapped in, unless you want the torque convertor to automatically lockup, a manually activated switch works just as well.

how do you wire the switch?

The manual switch merely takes place of the electronic switch inside the auto trans ECU. The switch applies power to the solenoid which locks the convertor

where do you wire the switch in?

  In order to replace a 5spd with a automatic. and want to do it right you will need to use parts from a pre 96 .you will need flex plate w/spacer for end of crank shaft,rear drive line,shifter assy.w/cables .there is also a shift lock on ignition switch,brake pedal assy.,wire loom, ecm[brain],kick down linkage on throttle body kickdown cable,interior shift consule.
     
      shifter hole in floor will have to be slightly modified ,gear ratio in 5spd. is 5:12 auto 4:63
     my suggestion is to sell your 5spd. vehicle and get an automatic.