Scientific literature on the effect of various wavelengths of light on degradation of ABS and similar polymers suggests the following:
1) Yellowing is accelerated by UV light, both UVA and UVB across the spectrum
2) Weakening is similarly accelerated by UVA & UVB but as the plastic ages and yellows, visible light also becomes increasingly damaging
3) Bleaching out of yellowing is due to visible light at the blue/violet end of the visible spectrum
4) Yellowing is accelerated by dark because, particularly in normal indoor lighting conditions behind glass, the bleaching effect of visible light exceeds the yellowing effect of UV light
5) Normal glass filters UVB completely but UVA only partially. Laminated glass/windscreen/windshield glass or UV-filtered glass completely filters out UVA also
The conclusion to be drawn from this is that in theory the best and safest way to reverse yellowing without damaging the plastic should be by exposure to bright light in the visible spectrum e.g. bright daylight behind laminated or UV-shielded glass.
Note that although the windshield/windscreen of a car is usually UV-shielded, side windows usually are not and the dash of a car in full sunshine might get VERY hot unless it is very well ventilated.
Searle, N. D., Maecker, N. L. and Crewdson, L. F. (1989), Wavelength sensitivity of acrylonitrile–butadiene–styrene. J. Polym. Sci. A Polym. Chem., 27: 1341-1357. doi:10.1002/pola.1989.080270418
The wavelength sensitivity of unpigmented 100 mil thick ABS exposed to sunlight and filtered xenon are radiation was determined by the sharp cut filter technique based on three types of photochemical changes: bleaching, yellowing and loss in impact strength. Bleaching of the yellow‐colored species formed in the processed material is caused by wavelengths between 380 and 525 nm with maximum color change by the 475–485 nm region. Photochemical yellowing is due to wavelengths between 300 and 380 nm with all wavelengths being almost equally effective. The spectral sensitivity based on change in impact strength shifts from the UV to the visible region as photochemical yellowing progresses.
Pickett, James. (2004). Reversible post-exposure yellowing of weathered polymers. Polymer Degradation and Stability. 85. 681-687. 10.1016/j.polymdegradstab.2004.03.008.
The commonly observed effect of weathered polymer samples continuing to change color in the dark was studied in some detail for polycarbonate, styrene–acrylonitrile copolymer (SAN), and various combinations with SAN–polybutadiene rubber graft (SRG) copolymer. All of the formulations became more yellow upon storage in the dark after accelerated weathering, and all behaved very similarly. There was a fairly rapid shift of 1–2b units (as defined in CIELAB color space) over the first two weeks followed by a slower, linear shift that continued for many months. This amount of color shift is enough to be visible to the eye in side-by-side comparisons. Most of the additional color generated in the dark could be bleached by exposing the samples to sunlight through a south-facing window. The samples usually reached the original color after 2–4 days of window exposure. Putting the samples back in the dark caused the yellowing to start up again, although the rapid yellowing often was not as much as the first time. Samples aged under natural conditions outdoors also underwent a small color shift upon storage for 5 months and substantial color shifts after two years.