The series resistors are in the fuel injector circuit. They are mounted on the firewall (in Buses and Vanagons) on the driver's side, next to the double relay. It's a metal case surrounding four resistors, one for each injector.
These resistors simply limit the current flow to the injectors. I only mention them here in case someone might wonder what they are. They can be tested with an ohmmeter for the proper resistance, and that's about it.
The Bosch book by Charles Probst says that on some other L-Jetronic models these resistors were superseded and the current limiting was built into the injector windings. Later injector windings are brass while the ones used on our beloved vehicles are copper.