Yeah I bought a unit called 'Cardefender' It is installed in line with the starting and hidden feature. Once the Immobilier is installed, the system will only recognize the signal from the small black fob. And only you know where the main unit is hidden, so once you pass the fob over it, you'll hear a small "Peep" and then you have 10 seconds to put the key into the ignition and start the engine. I put one unit in my 90 Silverado pickup and one in my 88 Blazer. That was about 6-7 years ago and it has been working fine ever since.
The only minor problem is showing your mechanic or Valet how to start the truck. It cost about a $100.