On a 997 the rear lid emergency release is under the left side rear taillight. You can use a small hook about 6-8" long (like an old coat hanger - wrapped in something to protect your finish) to snake/sweep around in there and grab. It'll be under light taped to bumper cover. Should be very similar on 987.
That said... your battery is in your frunk so not sure why you are trying to access the rear deck lid to fix...
Anyhow... Do you have a cigarette lighter adapter for power source you tried to use on the fuse box post? If so, go ahead and use in one of cig lighter ports in cabin. You should be able to get enough 'juice' to activate frunk switch (maybe even start vehicle). If you couldn't do thru fuse box you may not have been grounded correctly and I would try again or thru one of cig lighter ports.
Good luck!
