Please help. 1989 slk convertable battery flat. Can't open doors. Key won't go all the way in door. It's not mega freezing conditions. Can open boot easy. Can some one please help.

Its possible the lock barrel has a small plastic spring loaded flap inside thats stuck use a thin blade to dislodge it completely or spray a lubricant in the barrel

Hi - you might be best to call a roadside repair company.

A solution for this can be to lube up the lock. However another solution has been posted and I shall paste it below.

For those who have a dead battery and cannot gain access to vehicle and manual door lock(s) do not work, perform the following:

1. remove the right front wheel.

2. remove the wheel well liner.

3. buy or borrow a charged battery.

4. buy or borrow a set of jumper cables.

5. attach positive jumper cable to only the large nut on starter, then attach the other end of the cable to the positive post of the donor battery.

6. attach negative jumper cable to negative post on donor battery, then attach the other end of the jumper cable to any clean engine or chassis metal. You will know that you have a good ground when you see small sparks when making connection at the engine or chassis.

7. then use the remote to unlock the vehicle.

8. then immediately remove the negative jumper cable from the battery and chassis ground, then remove the positive cable from the battery and then the starter.

http://www.benzworld.org/forums/w163-m-class/1417771-diy-thread-collection-10.html#post9046145