Roxy,

I understand your pain when it comes to big dollar repairs, but owning three late model Mercedes I do ALL the service work on mine.

Not sure what part of the country you reside on-USA it sounds like for sure?

The part numbers you posted are not for a wiring harness, but for a physical electronic part. Here is what they look like:

1635401217 (163 540 12 17) Diaphragm Position Sensor on Brake Booster manufactured by GENUINE MERCEDES for $190.62 at KO Performance

0044310412 (004 431 04 12) Brake Pressure Regulator (ABS Pump) NOTE: For cars with Electronic Stability Program (Code 472). NOTE: This unit must be programmed after installation. manufactured by GENUINE MERCEDES for $1480.38 at KO Performance

If you replace the pump(the expensive part) you may have to take the car in to get it programmed into the Dealer or Mercedes Independent repair shop

Now, I did find a thread on a common failure the pump happens to be the brushes on the pump being the culprit to your problem, and there is a cheap fix.

C1401 ABS/ETS/BAS lights come on randomly - Benzworld.org - Mercedes-Benz Discussion Forum

I did a little searching here

Look at the stickies for info

W163 M-Class - Benzworld.org - Mercedes-Benz Discussion Forum

Some ABS outcomes:

BAS/ESP and ETS lights came on - Benzworld.org - Mercedes-Benz Discussion Forum

Need Advice ASAP! (sooner if possible ;-]) - Benzworld.org - Mercedes-Benz Discussion Forum

ANY TRICKS TO KNOW WHEN CHANGING ESP PUMP - Benzworld.org - Mercedes-Benz Discussion Forum

ABS, ESP, BAS and ETS lights and incessant beeping - Benzworld.org - Mercedes-Benz Discussion Forum

I searched using the words ABS Pump

Benzworld.org - Mercedes-Benz Discussion Forum

A cheap Factory Mercedes Benz parts that sells at wholesale is Caliber Motors

I deal with Omar at caliber. Email them your request.

Wholesale Parts Department - Mercedes-Benz of Anaheim is the #1 Parts Dealer in the Nation

If the parts prices are still expensive. Then the Mercedes Recyclers are the other alternative.

Best of luck,

Martin