You don't need the cats to keep the check engine light off if you buy a $40 O2 Simulator from www.o2sim.com.

Installation of dual output 02 Simulator on 1999 Mercedes ML 320:

Contrary to the instructions, the black wire of the 02 Simulator must be grounded to the chassis and not to the sensor ground. Otherwise, a P0400 error code (exhaust gas recirculation flow) lights the check engine light within 7 miles.
Access to the wiring from the O2 sensor can be had easily underneath the carpet at the fronts of each front door. Just slip a screwdriver under the carpet and pry it out from under the trim panel on top of the rocker/door sill and reach underneath and grab the cable. The rug is not fastened down and can be easily reinserted.
The 02 Simulator sales personnel furnished me with the wiring diagram showing the color coding coming out of the computer which was still valid at the access point under the rug.
On the right/passenger side, the red and brown sensor wires are twisted together. The brown sensor ground (287 ohms to ground) can be cut or ignored. The red is cut and attached to the O2 Simulator white wire. The blue and pink wires power the sensor heater and can be left in place unless you are installing a resistor. The blue wire provides B+ on both sides and can be used to power the O2 Simulator, hooking it to the red wire. I used an independent power source before ascertaining that the blue wires would be adequate.
On the left/driver-side, the blue and green sensor wires are twisted together. The blue sensor ground (287 ohms to ground, not 4 as in the instructions) can be cut or ignored. The green is cut and attached to the O2 Simulator white wire. The blue and white wires power the sensor heater, with the blue wire providing B+.
I installed the O2 Simulator in a central location and found switched B+ by popping out the panel alongside the ignition switch. However, I would recommend simply installing it under the carpet, as far to the side as possible. There is a black plastic channel covering an indent on the floor under the rug that would provide a recess for it. To get a good chassis ground, you will have to drill a hole in the sheet metal and thread a sheet metal screw. You will then have one white wire to run over to the opposite side. If it were me, I would buy two single output 02 Simulators and install one on each side to make the wiring extremely clean.
I attempted to install resistors and bent the copper pipe clamp that comes with them into a hook such that I could slip it under the black plastic channel and hold the resistor near the inside edge and against the metal floorboard. They get hot and I wished to conduct heat away and also keep the resistor from contacting the tubing that runs just under the sill. The resistors are unneeded unless you are removing the O2 sensors.
After installing the second resistor, I got a check engine light and pulled them out. I suspect the 20 ohm resistance of the rectangular white Radio Shack power resistors supplied by O2 Simulator was too low. The power rating of 20 watts was OK. If you need them, measure the amperage of a sensor heater with 13 volts on it and use ohms law to calculate the resistance under load. The resistance of a filament changes when it is hot.
The check engine light cleared itself when I removed the resistors. I had 320 miles on it before adding the resistors and added 100 miles since the light cleared.