I have a 1997 E300, the backing plate / splash shield things behind the brake rotors are all rusted out.

If you're parting out a W210 and can sell me these, let me know. Must be in good shape. Light surface corrosion is ok, but no crusty or rust-through.

Thanks

If this is built up like a W202 / R129 these can't be changed unless the hub is pressed out of the bearing ( and bearing changed as the bearing usually does not survive the operation.) or you cut / weld the backing plate

Have a look on car-part.com a nation wide inventory search of participating salvage yards. You might be better off getting an entire hub assembly from a southern states car and not paying much.

Found the part numbers for these in case anyone else is looking:

210-420-15-44
210-420-14-44

One for the right side and one for left.

I think you're right about not replaceable without pulling the wheel bearing. Not sure that swapping the entire rear hub assembly for a used one is practical though. If you're going to the trouble of removing the whole rear hub from the car, it's only a few minutes on the press to change out the wheel bearing.

Probably easier to unbolt the old shield and cut it to remove it. Cut the new one in half, then tack it back together with the welder, in position, and bolt it down.

Doesn't seem like a great design on these things. Other euro cars use a "C" shaped shield that can be easily R&R without taking anything else apart.

A couple of years ago I picked up rear hub assemblies with 50 K miles for $ 40 each just to have some spares, and this included the ABS sensor. This was at an all import full service salvage yard.

The rear hub assemblies interchange for a very long time, just be sure to match up the caliper mounting ears and with ABS sensor and without ABS sensor. Have a look at car-part.com , this is a nation wide search of participating salvage yards.