[frankc]

Kraftwerk,
If I understand your question correctly, you are asking if part 20 can be used in-line after part 6, and I assume you are asking because you would like to avoid replacing the A/C L & R control boxes (parts 4 & 5 in 1st pic) with the non-A/C boxes (part #18 in 2nd pic) that have the feeds to the L & R dash vents?

I suppose this could be made to work, but then realize that you will be feeding 4 vents from the right side port of the blower (R control box, center dash vent, L & R dash vents), so the air flow may not be that great. However, I suppose this is not too much worse than the OEM setup where 3 vents are fed from that same port (R control box/dash vent, center dash vent).

Let me know if you need any pictures to show how the various tube sizes match up.