Well that depends, if the bolt holder on the plastic is torn you need a whole new one. If it just fell off randomly (rare) I guess you can tape it/mold it/glue it back together, but you'd be better off just buying a new one. Get under your car and check it out.
98' BMW 328i - Jet Black, Black Leather, Alpine CDA-9831 HU
<span style='color:red'>For Sale:</span> Sony CDX-F5000 Xplod w/ Wiring Harness, iBook G3 900 Mhz 640 MB SDRAM w/ Original Box and Software + OS X Panther + Microsoft Office
(PM on any offers, Going on E-Bay soon ...)