Where the five-cylinder uses a thottle cable and vacuum-line mechianism to control speed, the TDI uses drive-by-wire. Some minor fabrication is needed to make the accelerator pedal work, but you get cruise control for free.
The Accelerator Potentiometer
The ECU expects a variable voltage sent from a potentiometer connected to the foot pedal. Volkswagen's assembly includes position switches, a potentiometer a suitable return spring, and the linkage to connect to a pedal. The only thing needed is to mount it properly so the full range can be utilised.
There are probably many ways to mount the pot. You could measure the distance the cable needs to be pulled (it's not straight trig because the radius is variable) and calculate if the stock pedal and cable could operate the potentiometer mounted in the engine bay. I'm told Volkswagen has installed them this way before on some platforms.
The T4 came stock with the 2.5 TDI in Europe utilising the pedal potentiometer, and I chose to imitate the stock bracket for my installation. The B4 Passat bracket mounts after cutting off the top of the pedal and drilling a few holes, but holds the potentiometer at a 45° angle— much too agressive for the placement. Sources exist in Europe for the stock bracket (and someone on the vwt4forum is making also them brand new), but I simply cut the bracket and welded it back on at a new angle. Unfortunately I forgot to switch polarity back after using flux-cored wire for another job and my welds turned out very ugly. I went over it again and got enough penetration that I'm satisfied structurally, but I'm glad this will be hiding under the dash.
The pedal itself is too tall to connect to the potentiometer linkage. It needs to be cut off and then the top of the Passat pedal must be welded on. I tried to cut as little off the Eurovan pedal as possible for ease of replication. The bit hanging off is from the Passat pedal to show approximate length and placement. Take care when welding to remove the plastic adjusting bits and not to block the edge where it will travel.