I've got my plan all setup for adding one.... http://www.marinenutz.com/products/H...ete%20Kit.html
That kit has everything you need. I'd install the heater-core below the radiator (I have this really odd space under the glove box, in front of the spotters seat and only accesible from under the bow seats.... there is nothing but empty space and stereo wiring there in my boat. I'd run the snake up and have that vent exit right next to the glove box -- add another vent under the drivers seat and one blowing on the spotters feet and be warm + cozy year round. Could be on the list of upgrades for next year.....
Former owner of a 1987 Supra Saltare. Current owner of a Malibu 23LSV.