You can pull the carpeting up on the drivers floor to get to the door vacuum lines. If I remember right it splits there for passenger front and the rear and then on the rear drivers floor it splits again for left and right. You can test it just by sucking on the lines with your mouth and finding which ones don't hold if you don't have a mity vac or something like that. Oh, in each of the previously mentioned places there are two sets of lines, one for locking and one for unlocking so you have to find the right one.

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k