The Mercedes ML320 SUV was manufactured between 1998 and 2005, when it was replaced by a revamped Mercedes ML Class SUV. The ML320 did not receive a redesign during its production cycle; thus, the procedure for replacing the headlight bulbs is the same in all model years. Installing new headlights isn't a backbreaking task, but it is quite tedious. Expect the procedure to last approximately 20 minutes per headlight.
Shut the engine off and open the bonnet. Walk in front of the headlight you want to replace. Examine the area in the inward corner of the headlight assembly. There is a plastic trim piece beneath the assembly; in the corner, you'll see a small tab. Depress this tab with a flat head screwdriver to release the trim piece; pull the trim piece outward to remove it.
Remove the two 10mm bolts from behind the trim piece (bottom part of headlight assembly), and then remove the 10mm bolt on top of the assembly.
Inspect the top-outward corner of the headlight assembly. There is another tab--similar to the one that attaches the front trim piece--that needs to be removed. Push in the tab with the flathead screwdriver to release the headlight assembly.
Pull the headlight assembly out a few inches. At its rear, you'll find an electrical connection. Press the plastic tab on this connection to unhook it from the headlight assembly. Pull the headlight assembly out fully to access the bulb sockets.
Press the plastic tab on the rear of the assembly to remove the bulb socket cover (low-beam). If replacing the high-beam headlight, twist the cover on the innermost part of the headlight assembly.
Depress the clip on the bulb socket to release the wiring harness. Turn the bulb counter-clockwise to pull it out. Insert the replacement bulb; turn it clockwise. Connect the wiring harness to the replacement bulb.
Push the bulb socket cover up to replace it (or twist the plastic cover clockwise, if replacing the high-beam bulbs). Re-insert the headlight assembly into place, leaving a few inches of clearance to reconnect the electrical connection.
Reconnect the electrical connection to the rear of the headlight assembly. Push the headlight assembly firmly into its mount (this will re-engage the clip). Tighten the three 10mm bolts with the socket wrench. Position the trim piece back in place; push it in firmly to re-engage its clip and attach it to the lower part of the headlight assembly.