First, you are wasting your money on the Dexcool by adding it to
conventional coolant.  It works fine as a coolant/antifreeze mixed this
way, but will provide no extra life, you must drain and flush all old
stuff out and replace it with 100% Dexcool to get the long life
benefit.  Given the corrosion problems that can occur when conventional
coolant is not changed every two years I think this is a worthwhile
thing to do.  Dexcool's anti corrosion chemistry does not break down
into acidic by products like conventional antifreeze.

It definitely sounds like the rear heater or valve is leaking.  Check
under it, but the smell is a pretty definite indicator.  Cores are
available from the list vendors for around $80, valves about half that,
and both are pretty easy to install.  If that's not it, look elsewhere,
you definitely have a leak somewhere!

Good luck!

--
Stuart MacMillan
Seattle

'84 Vanagon Westfalia w/2.1
'65 MGB (Daily driver since 1969)
'74 MGB GT (Restoring sloooowly)

Assisting on Restoration (and spending OPM):
'72 MGB GT (Daughter's)
'64 MGB (Son's)

Stripped and gone but their parts live on:
'68 MGB, '73 MGB, '67 MGB GT