I put a valve on all of my cars that don't have a valve. It's the next best thing to AC and the results are stunning.

I also put one on my SL because MB controls water leaving the heater core rather than water entering. And, . . .the stock MB electric valve defaults to open when car is off / power removed, this allows for thermosiphon when the engine is off leading to a hot interior on a restart.

I use a 3/8" pipe ball valve and a 3/8" pipe about 6 - 7 " long cut in half ( be sure to debur / chamfer the ends. The pipe OD is slightly larger than 5/8" but smaller than 3/4" allowing it to work with both sizes. Be sure to leave the pipe slightly rough so the hose has something to grab onto.

Given your heater control system isn't working and MB generally uses water flow to adjust temperature, give some of these below a look. Also have a look on Rock A and the Four Seasons web site, for cable control, use cars from the early 70's and dig around.

FOUR SEASONS 74627 $ 17 73 Ford Bronco
5/8" in line cable operated

FOUR SEASONS 74662 $ 33 72 AMC Matador
5/8" 90* cable operated

FOUR SEASONS 74809 $ 10 99 Ford Ranger
5/8" Inline vacuum operated ( apply vac to close ) w/ 4-Port Valve < used on systems that need water flow through the heater supply and return lines when heater is off.

FOUR SEASONS 74859 {#74622} $16 99 Ford Ranger
5/8" Inline vacuum operated ( apply vac to close ) w/ 2-Port Valve