Keyless install problems?

I'm working on installing a keyless entry in an 04 Trailblazer that came without.  I purchased the Commando KE-30 and a GMDL1 to interface.  The actual interface says it's a GMDL7, but still applies to the Trailblazer; it just looks like it is capable of doing more??  I got everything hooked up yesterday and all worked well except the doors wouldn't lock.  The doors would unlock, horn would honk and parking lights would flash, but no lock function.  I double checked all of my connections and tried hooking up the wires that tell the interface to lock or unlock differently and now the thing won't lock or unlock.  I think I might have fried the interface or the keyless while testing it, but I'm not sure.  I went and picked up another keyless locally and the thing still doesn't lock or unlock so I'm pretty sure the interface is bad.  How sensitive are these components?  Why didn't the thing unlock when first hooked up if everything was connected properly?  Is it common to have defective interfaces or did I do something wrong?  any comments or insight would be greatly appreciated.

Thanks,

Eric

I reprogrammed the module each time I unhooked it and hooked it back up.  The procedure for this module is to push the program button before the connector is inserted and hold while you turn the key to the on position, light blinks fast, release button, start vehicle and light turns off.  The module successfully did this each time I unhooked and hooked it up (which is supposed to mean it is in synch with the vehicles computer).  I tried locking with door open and door closed and still nothing.  Also, I windowed into the wire going to pin2 in the OBDII test connector.  I wonder if there's an alternate method to program/troubleshoot the interface?  My plan is to order another interface and hope the problem goes away but it would sure be nice if I could fix what I have (my wife would be VERY happy).  Any ideas on how to rule out if the interface is fried or not without replacing it?  I really appreciate the quick responses!

Thanks

I'm not familiar with the MULTIDL.  Where is the best place to buy and what's different from it and a GMDL1?  Thanks