Another speaker set up that works is speaker boxes hanging from the front of
the pop top.  My friends van has this set up.  He hung two 6x9's in brackets
from the front corners of the pop top.  The brackets were drilled directly into
the poptop and sealed with silicone so they didn't leak.  The speaker wire ran
under the carpet to the driver side seatbelt pillar, up the pillar, and then
into some curly phone cord type speaker wire that would stretch out when the
pop top opened.  This went to the speaker behind the drivers head, and then
across the poptop to the speaker behind the passengers head.  The speaker boxes
could rotate from facing forward or backwards.  Very cool.