I haven't done any audio upgrades on my car but from what I've read here it sounds like it's all or nothing when you upgrade the factory stereo. I'd go with the spoiler for now or save for a complete audio overhaul.
Summer beater - 2007 Mustang GT Convertible - Tungsten Grey - 5 speed handshaker, Eagle GT's, FR500S axlebacks, Airaid CAI, Bama tuned, Steeda strut tower brace, 5W-20 standard oil, stock hood corrosion and occasional fuel fill troubles (SOLD)
Winter beater - 2006 Acura TL - Anthracite Metallic - AT, Continental DWS, AM/FM cassette, non-navigation (SOLD)
Former summer beater - 2003 Honda S2000 - Loser Blue - 6 speed, no mods (SOLD)