Any ideas what seal I need?

[RichardFX]

It depends on where the leak is. One possibility is hoses can become detached

see here

What have you done to your MX-5 today? (Part 2)

Each side of the PRHT has a tubular drain from the rubber seal around the base of the back window, and a moulded plastic drain from the top of the B-pillar near the seat-belt guide. These discharge onto the grating which then exits through the flaps to under the car. [20200810_141715_d] Looking up at the back window drain tube connection [20200912_232450_c] I’ve not tried to remove the gratings yet, a good strong wet-vac underneath the car held over the outlet with a hand making the seal …