Assistance in diagnosis

Hi all,

I come to you all in need of some assistance.
I apologize for the general post title. I've found that being too specific makes people not want to click in and read.

WARNING: Lots of words ahead. Summary available at end of post.

I have a 2006 package 6 (upgraded stereo + NAV) to which I installed a VAIS SL2Vi. This added iPod integration in the form of an emulated CD-MP3 changer. It also added video input from the iPod to the NAV screen via emulated DVD player. It works wonderfully.

Recently I've installed the Coastal Ultimate Lockpick 3+ (which adds the ability to somewhat bypass speed restrictions on the NAV and radio through the use of clever circuitry to selectively and temporarily "disconnect" those purple speed sensor wires to the various ECUs and other components.) It also included extra harnesses to add RCA input to the radio/NAV screen.
The first feature, again, works flawlessly. The video input feature does not.

Now, when I was ordering the ULP3+ I did some research and did determine that there would be a conflict with the SL2Vi. However, none of the places (both VAIS and Coastal) offerred any information other than "We do not test our products for compatability with competitors' products." Fair enough. When pressed for more information, "off the record," the really could only tell me that there was indeed a conflict between these specific two devices, but didn't go into more detail than that. One company offerred the solution of just unplugging the audio and video harness of the ULP3+ without explaining why, or what the conflict would be.

Oh well. I took the plunge and ordered the ULP3+ knowing to only expect the NAV and radio overrides to work. And they do.

BUT, being as curious as I am, I had to figure out what this "conflict" was. Turns out it's isolated to the radio, specifically the video signal of the DVD emulation input. Great, so nothing important in terms of the main functionality of the two devices. Moving on, I determined that plugging in both the ULP3+'s RCA video adapter harness with the SL2Vi's video harness results in no video being sent to the NAV screen. Menus and overlayed settings still work, so I conclude that the radio and selected mode are functioning as intended. The problem lies with the add-on devices and their wiring. Audio works fine from both sources, so it's just video with the problem.

I'm going to try the unofficial advice from one of the companies earlier, and unplug the video harness from the ULP3+ to see if the SL2Vi's video still works. I'm thinking it will. Then I will try just plugging in the ULP3+'s video harness, removing the SL2Vi's and see if that still works. I'm thinking it will, too.

Now, the main question I propose to all of you brilliant Priuschatters: Assuming these two methods still work, how can I go about making the two harnesses work together?
I think they plug in series (I'll have to take another look), and I'm thinking the RCA connector is leaving an open circuit when nothing is plugged into it, therefore also preventing the SL2Vi's signal from getting to the screen. Would a passthrough RCA connector work? How would I even build one? Does this even make sense? My knowledge of RCA is limited to "signal in the middle, ground/shielding on the outside." But of course, I have no idea what I'm talking about, so please correct me if I'm wrong.

I'll be sure to come back and update after I've tested the isolation of the harnesses.

Thanks for your time.

tl;dr: Installed VAIS SL2Vi and Coastal ULP3+. Video input conflict. How do I diagnose and fix?