here is a guide, i have used it many times

Key synchronization Procedures

SL Models
90-95 see note 1
96 see note 3
97 see note 2
98-99 see note 4

S-Class Models
92-95 see note 1
96 see note 3
97 see note 2
98-99 see note 4

C-Class, CLK, E-Class
96 See note 3
97 See note 2
98 –99 see note 7

SLK

See note 4

M-Class

Early 98 Models up to A010609
With original AAM and keys see note 5

Later models 98-99 after A010609 see note 6

Note: All M-Class keys require the HHT for Initial Installation

Note 1: Aim transmitter at a receiver and briefly press any transmit button. Within 10 seconds, lock or unlock car at a door or trunk with the new master key.

Note 2: Aim transmitter at a receiver and briefly press the transmit button twice. Within 30 seconds, insert new key in steering lock and turn to position 2.

Note 3: Aim transmitter at inside rear view mirror receiver and briefly press transmit button twice. Within 10 seconds, insert key in the steering lock and turn to position 2.

Note 4: Aim transmitter at vehicle and briefly press the lock or unlock transmit button twice. Within 30 seconds, insert key in steering lock and turn to position 2.

Note 5: Key to position 2 (ON position), then turn off and remove key.
Hold down "lock" button, and push "unlock" button 5 times within 10 seconds. Then release buttons and push either the lock or unlock button to confirm operation

Note 6: turn key in steering lock to position 2, then to position 0 and remove. Within 10 seconds, press the lock and tailgate buttons simultaneously for 15 seconds.

Note 7: For Smart key equipped vehicles, simply insert key in the ignition switch and follow info in the cluster display