If you've just bought a new GM fullsize truck, an Escalade or a Hummer H2 with a big double-DIN-sized head unit screen in your dash, and you want to upgrade your stock sound system, do your homework first. That's because the automakers are increasingly tying the electronics in the head unit to data control circuits in the vehicle, to be tweaked at your peril.

What exactly is the databus, and why is it in your truck? About a decade ago, the automakers discovered there were huge dollar and weight savings to be had by using digital technology to send multiple function control signals over a single wire. A central computer connected by a single wire to a node in your door can send commands such as lock/unlock, windows up or down, and door trigger back and forth along the bus.

There are various classes and types of databus. One of these is called J1850 (or Class 2); another is called CAN (for Control Area Network). Some of them use copper wire, but the next big thing is fiber optic. Since there is no industry standard, the only thing these databuses have in common is that they are currently not very friendly to aftermarket upgrades.

"This is an issue that hasn't really been widely recognized or addressed," says John Wilson, Audio Research Support Specialist for Directed Electronics. "It's sent installers on a wild goose chase when factory head units were removed and their data path was interrupted. The trucks in question can lose secondary control functions like domelight, door chime, and door trigger, and suddenly a simple head unit install can become a hopeless mess."

While there may be some who find the stock sound system is adequate in these trucks, the majority will likely sense there is room for improvement. In particular, the factory head has notable limitations as the source unit for a high-performance audio system. Retaining the factory radio may result in unsatisfactory sound even when aftermarket speakers, subwoofers, and amplifiers are added to the mix, with midrange and midbass frequencies that are unnecessarily suppressed by the factory radio's signal clipping protection circuit. In the past year or so, this has sometimes led to tension between mobileelectronics retailers and their customers, who were hoping for a better result.