Most likely during the disassembly of taillight bulbs, one of them got whacked; a "fast-blink" is a spot-on indicator of a broken filament somewhere in that system.  And, since left side works fine, just match up right side (with new bulb).  It wouldn't hurt to shiny up contacts with steel wool on sockets, and I recommend a wipe of Vaseline too for long term success (corrosion protection, and makes bulbs easier to remove next time).

    On Sunday, June 11, 2023 at 02:17:35 PM PDT, John Lauterbach <john@jhl.mgacoxmail.com> wrote:  
 
 I finally got some time to work on my 1991 Vanagon.  I had parking light and
taillight assemblies to install as well as a pair of new power mirrors from an order
I had placed two years ago.  When I had the right mirror loosened from the door, I
realized that I could not do anything without removing the inside panel from the
door.  Also, I had not checked the fuses.  I got my Bentley out but could not find
anything in the electrical diagrams that covered the 1991 model that showed me what
fuse I would need to check.  So, first question is what fuse protects the power
mirror circuit.

When i put the new taillight assemblies on, I apparently scrambled which bulbs went
where.  The left-hand side went perfectly, but on the right-hand side, I found two
single-filament bulbs and two double filament bulbs.  Bulbs were Osram, but I could
barely read the writing on the base of the bulbs.  I put the two double-filament
bulbs in the taillight and directional light sockets.  The right-directional signals
flash quickly, instead of at the normal rate. I suspect the answer here is to make a
trip to the FLAPS in the morning and get all new bulbs.

John