Hi Roman,
If you have air-conditioning, like I do, getting to the heater valve is very difficult. If you still have the dash apart, and have access to a syringe, squirt some PB Blaster in the cable and let stand for a while. Then carefully try to move the cable in and out. When I replaced my levers a few weeks ago, my heater valve was slightly sticking. After some PB Blaster and some careful in and out of the cable (prior to installing the new levers), the valve and entire cable slide like butter. Another option in place of a syringe would be to go to a local motorcycle shop. They sell a small device that clamps to the end of the cable and allows for the small tube of any standard can of lubricant to be inserted so that you can direct the lubricant spray into the cable. Somewhat more messy than the syringe method but would also work.
Good Luck!
Bob
bpossel
Memphis, TN.
1971 280SL
1997 E320