You might still be able to use the other sensor mat, depending on its range of resistances. If the resistance is too high, you can always plug up another resistor in parallel to achieve less resistance. The formula for resistors in parallel is:
The thing you will have to worry about is the range. You may get the resistance right to simulate no passenger, but once a passenger is added it may be way off. If you can give me the measurements of the
stock (2004+) sensor mat with/without an occupant I will know what the car is looking for, and I might be able to rig up a circuit for you. I've got the numbers for the 2003 mat already (shown above), so if you can plug up a multimeter to the stock mat, I might be able to make it work.