Welcome and unfortunately the trim can not be replaced as it is part of the glass. There are some options as I have painted them with SEM trim paint. Sand and prime a few times to get it nice, smooth and even. Throw a few coats of paint and bam it will look 90% to 95% better. Or you could get new glass.......see below.
Mustang Replacement Quarter Windows (87-93) Hatchback
Mustang Carlite Style Glass | 1979-1993 | NPD
2016 VW GTI 2.0 Turbo
1989 Sonic Blue LX Coupe, 349 ci, TW heads, TFS Track Heat Intake, 75mm Accufab TB, 80mm ProM, 30 lb. inj, TKO, 3.73, 31 spline w/ TruTrac, MAC 1-5/8" LT headers w/ H pipe, Flowmaster, electric fan Moates QH
391 rwhp & 424 rwtq...Mustang Dyno 11.5@122mph