I second Eye Roll's technique. You really, really don't want to remove the heater box, which must come out if you need to remove the bottom mat.

However, the leak may be from the air vent in front of the windshield. That vented bodywork panel between the hood and windshield is held on by a bolt (10mm, I think) in the middle, as I remember. You reach it from the engine compartment. Remove the bolt and the panel slides forward and up to be removed. You can then see the channel that holds the wiper motor and the heater intake. Look carefully for cracks or leaks: this area is just above the firewall on the interior side. Any crack will leak water onto your rug underneath the dash where you can't see it.

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-- Bruce / '82 244 (315k miles!), '86 745, '87 760Ti, '94 854