I recently had my battery tray out. It didn't have much rust and there was none on firewall. But that is probably because I painted tray and firewall with POR-15 in early 90s. Only needed touching up.

Most new batteries have a vent. On my E320, the battery is under the back seat near a lot of electronics. The battery vent tube goes through a grommet on the floorpan. Thought passed my mind that on 300D, I should run a vent tube down through the engine compartment to under car. But there doesn't seem to be much of a problem so haven't done it.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5