I don't know if this can apply to your 190D, but when I had the problem on my 300D, I would jiggle the steering wheel
for about 30-45 minutes while simutaneously trying to get the key to turn with ease.
It takes time and patience.
Anther trick was to insert the key with the front tip slightly pointing down and applying that angle of pressure while turning.
It a mix of the above that finally got mine going so that I could replace the whole set up.

russian roulette!!

__________________
1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
********************
1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD