MVS forums member burnout8488 writes a very good DIY piece on fixing a cracked heater hose at the firewall:
burnout8488 »
I, like many, had a cracked heater hose junction at the firewall. This came in a lot cheaper than a new one 
The coupler needs to be gutted to make room for the hose clamps that will be housed inside it.
Slide the metal trim plate over the metal hoses, then connect 5/8″ heater hose to them and clamp it down. The plastic junction shell needs to be inside the cabin at this point. Screw it in using the T15 Torx screws and the engine bay side is complete.
The aluminum heater pipes need to be cut to make room.
Cut the hose to the proper lengths in the cabin (total guesswork, it’s so cramped), install the pipes onto the heater core, and clamp it down! Worm gear clamps will NOT work here, as there is no room between the gas pedal and the heater hoses. You will need to get some clamps like the ones shown. I stole some off of a family member’s Corolla. 
Your gas pedal mounting bracket may need to be trimmed if using different clamps. Nothing a Dremel can’t solve.
So far it holds pressure and doesn’t leak a bit! I have confidence this will last a very long time.
Your gas pedal mounting bracket may need to be trimmed if using different clamps. Nothing a Dremel can’t solve.
Heater Hose Firewall Coupler Elimination w/pics
But wait. There’s more.
JimBee also approached this fix from a non-orthodox angle, and had success. Read his outstanding DIY:
JimBee »
Volvo 850 / P80 heater hose hack: how I did it and some other options.
These are more like craftsman’s projects than a mechanical fix.
After getting a messy leak there, I wanted to do away with the junction and all 8 of its o-rings and go all the way through the firewall with standard heater hoses.
I wanted to do the mod in a way that prevents kinking the hoses at the firewall and without stressing the pipes out of square where the o-rings seat at the lower end into the core manifold.
Eliminating the junction, of course, means you need a way to secure the face plate for the hose pass-through on the engine side of the firewall, tightly against the firewall. For that, I made a plywood backing plate that you’ll see below.
Read the rest:
