The Basic Stamp has a function called PULSIN that will give you the width of a pulse on one of the input pins. You can sample multiple pins (input pulses). You can put these 3 PULSIN functions in a loop to keep sampling the PWM inputs. This can be a call in a larger loop that does other things. It shouldn't take much of the CPU power.

The way I do it is to route the RX PWMs into the Basic Stamp then let the code decide if it merely duplicates the pulse in software to the appropriate servo (manual mode) thru an output pic, or let the code take over control (auto mode). No relays or switches to route PWMs, the processor does it all in software.

A note of caution....the Tx/Rx that comes with most (inexpensive) RC cars is not a typical system that you would get if you had a regular RC Tx/Rx system (3 wire standard servo cabling). I ripped all of the truck controls out and replaced with a standard Tx/Rx system (Specktrum DX-7). Not to say you can't make it work, but you better have an oscilloscope and a lot of patience.

The other advantage of a real system is you have more channels to use for more features. One channel can be to tell the car if you want manual or auto for instance.

As I mentioned, I migrated to the PIC from the Basic Stamp to get more power and capability...but still think the Stamp is a good place to start.

John