GE's site has an extremely thorough interactive automotive lamp selection guide here: GELIGHTING.COM
Mustangs Unlimited carries this lamp as their p/n: TL631
Per GEs site, input 631 in the search field and its specs are as follows:
G6 Lamp - Single Contact Bayonet (BA15s) - 14Volt. (1000 hr. Rated Life)
However, when you use the specific vehicle search feature it specifies an 89 lamp for the trunk:
G6 Lamp - Single Contact Bayonet (BA15s) - 13Volt. (750 to 1500 hr. Rated Life)
or a 97:
G6 Lamp - Single Contact Bayonet (BA15s) - 13.5Volt. (5000 hr. Rated Life)
I'd go for the 97. The 0.5V difference might have a very slight affect on lamp life, but considering the 5,000 hr. vs 1,000 hr. I'm sure you'll still come out way ahead.