The turn signal flasher relay expects a certain amount of amps to flow
when the blinkers are on. That way if one of the bulbs burns out it
blinks faster to let you know. If you put the wrong bulbs in and not
enough amps flow then it can blink fast even when all bulbs are lighting
up. This is pretty common in Vanagons for a couple of reasons.

First, the front signal bulbs are dual filament types. It has a dim
filament for running lights and a much brighter filament for signals.
That way the blinker lights can be seen blinking even when the running
lights are lit. The flasher relay expects to blink the brighter filament
and if things are wired so it blinks the dimmer one then it sees a
problem and blinks fast. The 2 outer wires on the front socket may be
plugged in reversed and this reverses which filament is for the blinkers.

Second, the dual filament bulbs come in both polarities. That means you
can buy what looks like the right bulb but it will blink fast because
the wrong filament is being used for the blinkers. You can either get
the right polarity bulb or swap the 2 outer wires on the front bulb
socket so the wiring will match the bulb polarity.

Another fast blinking issue comes up when people try to use LED bulbs
for the blinkers. The LED bulbs use a lot less amps and this causes the
normal blinker relays to think there is a problem and blink fast as a
warning to the driver. This can be overcome by getting a flasher relay
designed for LED bulbs.

Mark