just noticed a few drips of brown liquid on my passenger floormat, felt around the heater core seems like it is leaking from there. anybody got any info?? 1966 coupe

 

Most likely it is your heater core. Take the plenum off so you can get a better look at it. Start the car, and turn on the heater and see if the core is leaking or if one or two of the hoses is leaking. If it's the core, then order one from a mustang vendor and replace the old one.

 

yeah, and be glad you don't have a fox! had to do one on my 89 and it was a disaster! the center consle had to be removed followed by the dash board than after taking off every bolt in sight, i still had to cut the box open to get to the core! thank god for JB weld!!! ended up driving around for 3 weeks with the dash and center console sitting in the spare bedroom!