the worst rattles in my mk3 were
bootlid
front doors
roof
wipers
the rear quarter`s were ok.
will be worth trying to tighten your boot catch a little to stop if rattling but it means you will have to slam it a bit harder;)
other than that just put it in a few places and see where the next rattle comes from.
flashing tape works but is nowhere near as good as proper soundproofing but then normal dynamat isnt great ideally you should use dynamat extreme or something like second skin products.
