For a test, remove the two hoses from the heater core in the engine bay and connect them together to bypass the heater core. You can even flush all the coolant out of the heater core with fresh water and air. Drive like this for a couple of days to see what happens -- don't run the heater blower. Sounds like your mechanic is the kind of guy to work with you. He should be able to do this in less than 1/2 hour if you are not up to it.

Typical symptom for heater core failure is that the windshield fogs up with coolant when running the defrost.

I just replaced the heater core on a 740 and a 760. There are good instructions in the FAQ for the740 which is sightly easier than the 760. This job isn't for the faint of heart, not rocket science, just a lot of twisting and turning in uncomfortable conditions. It is very expensive if you have it done as it requires 8 to 10 hours of labor. I suspect your mechanic is suggesting to look at the lower duct distribution box to see if there is any coolant in there. You may be able to get into this by removing the passenger knee panel and the vent into the footwell on that side. If you can do this yourself, reach into the vent opening and feel around to see if there is any trace of coolant.

Good luck,

Dan