My 1999 300m has trouble shifting into 1st usually in the morning when it is cold outisde goes into reverse fine but then to first it takes about a minute to shift

Could be TCC solenoid, or something similar. Check computer for trouble codes. Even though it's transmission related, it will still trigger a check engine light and store a code in the ECM.

sound like u might have a vaccum leak

1st off has the tranmission fluid and filter ever been replaced? Do that first.

What year and model? Sounds like the one way clutch is broke in the trans. It will need rebuilt.

Every car needs a heating engine time specially when its cold, I recomend like about 5 to 10 minutes after you start engine and before you change from P to D. If you try to start the engine first time in the morning, it's cold outside and you immediately go on run you are going to have this trouble every time. Every engine has it's operational standard temperature a 5 to 10 minutes working engine, car stopped, may guaranty this standard temperature and optimal function conditions, and also keeps a longer life of your car.

Good luck

If you have a fluid clutch then try bleeding to see if any air is in the pipes.If its cable check the adjustment.If all is fine then you have a clutch pressure plate fault.Solution Replace the clutch assemebly

try a full flush and filter.
hot ATF fluild flows more easy thru clogged filter.
or bad vac modulator or governor or its tiny filters.

you need a AT expert , if the flush dont work.
there are many on car tests.

When you first start driving it usually takes atleast 4 miles to warm the transmission to operating temperature. The temp of the motor may say it is warmed up, But, the trans fluid and differential fluid only begins to warm as you drive and the parts are actually working. THe computer has sensors that keep the trans from shifting to OD until the temp is up as not to add stress to the motor until warmed properly. Hope this helps and have a good day.

try tranny oil change and add lucas atf tranny fluid

How old is the gearbox oil? In some cases, older oil (higher viscosity when old) will be too "thick" in colder weather and inhibit the synchro on your gears from slipping into 1st gear. The reason it only happens in 1st, is 'cause the shifting mechanism on the forward change is finer than that of the reverse change.

POSSIBLE Solution: Replace the gearbox oil with a lower viscosity ie. "thinner" oil.