Is there any way to open the trunk of a mk4 (Jetta or Golf) without a battery, in the case of a junkyard, or do I have to have a battery connected, so I can use the switch on the driver's side door? Of course, keys are not an option in this case.
Opening either from the outside is tricky, and "picking" the lock may provide hours of frustration.
Federal mandate on emergency trunk release:
https://www.federalregister.gov/doc...hicle-safety-standards-interior-trunk-release
Jetta: All cars with trunks were required to have an internal emergency release starting Sept 1, 2001. If the Jetta was built after this date, there
should be a glow in the dark emergency release on the inside of the trunk wall. Having spent lots of time at salvage yards, the relationship between the build date and emergency release is not always exact. Access to the handle would be by lowering the rear seats and climbing through. If there is no emergency release, the felt covered panel needs to be removed to access the lock mechanism. IIRC there are 12 tiny phillips screws (each recessed and covered in felt).
Here's a not so great DIY:
https://www.pelicanparts.com/techar...placement/47-BODY-Trunk_Latch_Replacement.htm
Golf: Since the Golf does not have a "trunk" per the NHTSA, the MkIV does not have an interior emergency handle. The Golf hatch mechanism is easier to access than the Jetta, and again requires lowering the rear seats and climbing through. The hard plastic cover needs to be removed, and then pop the mech open. BMAC is a British VW guy who has lots of good DIY videos:
How to Open Broken Trunk Lock on a VW Golf 99-06 Simple, Easy, Steps:
https://www.youtube.com/watch?v=Iv0kST3OzrY
Salvage Yards:
If you are at a pick-a-part, check all the nooks and crannies for a key. Although some large yards (LKQ) attempt to collect keys and zip tie them to the steering wheel, they aren't always available (or bad customers hide/take them). The key
might get you into the trunk.
Many good salvage yards have batteries available for you to use to test components. Many of the west coast yards have wheel barrows with battery ready to go for customers checking electrics or lights, etc.
Some yards may have no system for storing keys, don't leave the keys with the car, or they don't want you to attach a battery- and I've been to many yards where it's not uncommon to find the occasional locked car. In this case see the attendant, climb through, and open from the inside.