Kenworth T680
.tdealer.kenworth_4.t680 is configured without clearance lights.
All instances of cab variant A (so-called duty cabin) can be configured with no clearance lights if neither a Cabin Light nor a Roof Deflector accessory is assigned (or if the player removes them as neither is defined as required[]).
Suggested solution: Mark the cabin light accessory group as required[], introduce a 'blank' cabin light accessory with price: 0 (so it doesn't show up in the store), and have the cab deflector conflict with the 5 and 7 cabin light accessories while setting the 'blank' one as its default.
Kenworth W900L
.tdealer.kenworth_4.w900 is configured without clearance lights.
All instances of cab variants A, B, and C can be configured with no clearance lights if no Cabin Light accessory is assigned (or if the player removes them, as they are not defined as required[]).
Suggested solution: Mark the cabin light accessory group as required.
Peterbilt 579
.tdealer.peterbilt_4.579 is configured without clearance lights.
All instances of cab variants A, B, and C can be configured with no clearance lights if no Sun Visor accessory is assigned (or if the player removes them, as they are not defined as required[]).
Suggested solution: Mark the sun visor accessory group as required, or introduce a cabin light group and use the method suggested for the T680.

Clearance markers are required lighting equipment under FMVSS/CMVSS 108 and so to someone accustomed with North American vehicles, their absence is as visually jarring as, say, if the trucks were missing headlights or brake lights.