I did this a while back, using Ebay seat covers and a set of seat heaters from Amazon. I have an auxiliary fuse box in the passenger compartment, so I used two 10A circuits for the two seat heaters.
Installing the seat cover itself is straightforward, sleepydad has a great video tutorial.
One thing I would wholeheartedly recommend is ditching the '96-00 Limited lumbar switch, which I don't believe is intact on any 4Runner. I bought a plastic trim piece and matching switch at a junkyard from a '01-02 Limited, which is much sturdier:
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1997 4Runner Limited 4WD E-Locker ~200k | Falken Wildpeak A/T3W 265/75/16 | Pro Comp 69 16x8 | OME 2906 | B&M 70264
Addicted Offroad Front Bumper | Spiker Engineering High-Lift Hood Struts and Ultragauge Mount