I don't know why you couldn't use an inline type heater. I think diesel giant has a tutorial on the installation and a parts link. Just pretend it's an old MB you are working on if you choose to go that route.
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If it ain't broke take it apart and find out why.
1983 300SD, 4 speed
1994 C280
1987 300TD wagon
1996 HD Road King
Ride in Peace Eric Peterson, Harley of Macon