It sounds like a connection issue. When you replaced the bulbs some of the harnesses could have lost contact, or the individual pins in the sockets I have the same year and I’ve had my fair share of issues.
I’d start by cleaning everything off and put some dielectric grease in with the bulbs. If there’s any old hardened grease remove it. Retest.
If this didn’t work, I’d recommend turning the lights on and manipulating the individual bulb sockets gently and around the wire connections a bit to see if anything comes on. Sometimes, but especially in the reverse lights, the heat can cause the plastics to warp and loose contact.
If that doesn’t work, and I doubt this is it, check your grounds. Specifically on the top of the block, front right, just below the alternator. There’s an electrical ground there that can wreak havoc, but you’ll usually pick up flickering gear lights from this.
Word of advice in the long run, switch over to LED. I have LEDS and HALO lights and I love’em.