I installed a prostart remote starter with a universal bypass transponder. it cranks but will not start.

Check the fuses.

The only 2 pin connector is for the transponder bypass module.
These wires are for the following outputs
2 Pin Transponder Output Harness #1123292
1) Red Battery (+) 100mA
2) Black (-) Output While Running 100mA

This connects your transponder key module to the starter, it activates the module when cranking to simulate a key.

a: When selected for while remote start on, the Black wire will be active at the same time ign. 3 becomes
active, and will remain active all the time the unit is operational under the control of the remote start.
b: When selected for output during the start sequence, the Black wire will be active at the same time Ign.
3 becomes active, and will remain active until the vehicle has started. This will be used for one time read
transponder circuits.
c: When the unit is selected for until ignition off, the Black wire will become active at the same time ign.
3 becomes active and will remain active even after the key take over and until the ignition in the vehicle is
turned off. This will allow the unit to be used for continuous read transponders circuits

I have a 2006 Rendezvous and I have the Prostart CT-4311TW model and I have the lock/unlock and hatch working but cannot start the vehicle remotely. I too think its the bypass but I am not sure which wire is the ground. I have the DBALL2 bypass model. Any help would be appreciated.

If you are sure that everything is connected properly, then the only explanation could the ground when running from the remote starter to the bypass. If you are using data to data(D2D), switch over to wire to wire. Test the ground when running connection from the remote starter (white) to the ground when running connection of the Xpresskit (blue). You could even go so far as to connect the ground when running of the bypass to constant ground for the test. You CANNOT leave this wire connected to chassis ground.
Also, that vehicle has a second ignition wire(white) that MUST BE power up for remote start. If not, it will trigger the check engine light.

You have the wrong bypass. The bypass you are using is for universal transponders. You need the Passlock 2 bypass. The passlock is a resistance that is read from your ignition cylinder by the bcm that will allow your truck to start.

in the key ther is immobiliser that the car must fill to start

check to see if the security light or anti theft light flashes. if that is the case, sounds like the bypass for the remote starter may be causeing the no start by tripping the anti theft system. Remove remote starter and bypass system and join any cut wires back together that may be cut for remote starter. Try starting, if theft light still flashes leave key in the on position after attempted start for 15 minutes till theft light goes solid, turn off and try again. Repeat process for three times to re-learn factory theft system in vehicle. If problem still persists, then take to your local dealer to reprogram factory theft module in the cluster.

use the 791 and wire it up for VATS, you will not use the loop at all

put your key in the ignition switch leave it in the off position, press the remote start button, if it starts and you did not install a transponder bypass modual in it you will require one, if you need one let me know as I stock this item

First you will need a transponder bypass modual for a 2001 dodge caravan with MUX output, if your ignition key is grey, you may also need a total of 6 resistors for this van depending on what features you want and at least 2 relays, then just follow the install diagram, if you run into ant problems you know where to find me, I have done dozens of thses vans so I know what will be required, if you need the bypass modual or any of the 6 resistors let me know