I don't mean to be a pain with this, but if anyone can help I would greatly appreciate it.
You say you are trying to 'ground' an input to the ECU that has a 5V pull-up. You say you need "zero resistance to ground" but that makes no sense in a digital logic circuit. If the ECU uses 5V logic it should be sufficient to get the pin down to, say, 0.25V to ensure it is read as LOW. WIth a fairly hard pull-up resistor of 1K Ohm you'd only need a contact resistance on the order of 50 Ohms to ensure the signal is read as LOW.
Perhaps the 'signal' is actually a 5V power line. Get yourself a multimeter and measure the current coming through that pull-up resistor. If it is delivering more than 20 mA then maybe it's not intended to be driven low. If it is delivering less than 20 mA you should be able to connect it directly to an Arduino pin and drive it LOW as needed.