There is indeed a heater return pipe that goes around behind the engine. This is far more likely to leak than a frost plug or head gasket. You really need to identify the exact source before assuming anything. We cannot possibly know where your leak is, only you can do that be carefull examination. Use a good light and maybe a mirror and get a good look in there.

A rear frost plug would be no fun but very unlikely assuming you have antifreeze in the coolant.

David Hunter

Definitely sounds like heater hose.
You'll find two going into the firewall near the brake booster.

Squeeze and wiggle them to find out which one it is. Read up on them in the 700/900 FAQ - most importantly do NOT cut hard through either hose against the metal connector - scratching it will make it leak later.

The one going to the head, below the intake, is a bit easier to replace than the one going to the red pipe. However both qualify as a true PITA. If replacing the one to the head, I recommend removing the idle air control to open up access under there. It has two fat hoses and an electrical connector. Easier than pulling the complete intake.
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Sven: '89 245 NA, 951 ECU, expanded air dam, forward belly pan reaches oem belly pan, airbox heater upgraded, E-fan, 205/65-15 at 50 psi, IPD sways, no a/c-p/s belt, E-Codes, amber front corner reflectors, aero front face, quad horns, tach, small clock.