The black box trailer converters contain the 4 diode solution referred
to by a previous poster. I have made my own a few times using at most $2
worth of parts from Radio Shack. 4 diodes, wired in a "W" configuration,
is all it takes. For the trouble involved, buying one for $10 is the way
to go. The small voltage drop caused by the diodes is pretty minor
compared to the drop from the long wire runs and poor ground returns on
many trailers.

Mark

VWRuss wrote:
>
>   Well... my test confirmed that this does not work - sigh.  Unfortunately
> I didn't decide to build one for grins but since the one I had from U-Haul
> didn't work.  I ended up with no right turn signals after installation, so
> I double checked the trailer and car wiring and found it to be (at least)
> as it was in the instructions.  I swapped the right and left turn signal
> leads and no left signals!  Since the manufacturer of the thing just molded
> plastic over all of the components, I've no way to see what might be broken.
>   Anybody have any suggestions on where I might get one besides U-Haul?
> VWRuss