This is a 1998 van. It has less than 70K miles on it and works very well except for little flukes like this.

l would try a voltmeter to see where you are losing power, then you could at least localize your problem.

Well bec some of these types of bulbs r dual bulbs it maybe hard to see the problem just by looking at the bulbs.Also the fuse may look good but not be good at all bec it is internally faulty.
What I'd do is try a brand new fuse same amperage or a fuse u know is the same & works , plus I'd try a new set of bulbs throughout the brake light & marker system.

Hope that helps & best of luck.By the way u do not mention how old the vehicle is & the problem could be in the brake paddle switch or in the wiring harness or ground wires to the bulb sockets.

Check the brake light switch above the brake pedal. It could be bad or just fell out of adjustment. l am assuming you checked the fuses and lights with a test light so check the brake light switch with the same test light. There should be power going in on one side and not the other until you push the button up, then there should be power at both wires. If not then replace the switch. GOOD LUCK!!!

It may be a short in the 3rd brake light. The one above the lift gate. We have had some issues with that. There may have been a recall on it.

Does the CHMSL (third brake light) work? If yes, the problem is likely the hazard switch (part of the turn signal-multifunction switch). Hold the brake pedal and wiggle the hazard switch while watching for the brake lights to flicker. If the CHMSL also does not work, the trouble is likely the stop lamp switch.

Its probably the brake switch- try disconnecting the brake switch and short the 2 contacts of the connector and see if the brake lights come on- if they do its the brake switch.