I am supprized that your 99 E300 does not have child proof door locks. My 98 E320 has child proof door locks on the back doors. They are activated by the switch located between the left side window switches on the center console. Double check your owners manual to verify if you have child proof door locks.

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Ray
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles