What was the first modification that you made to your Mustang, Lightning, Raptor, etc.? Did you plan on modding your vehicle or did one thing lead to another?
Are we talking the latest Mustang. If so, the first thing I did was Redline hood struts.
I have a habit of buying stuff when I find a deal and taking forever to put it on the car. Case in point: I have a brand new pair of Flowmaster 2-chambers I bought in 1993 and never installed
Ironically, and not because the OP is from Flowmaster, but on the 2014 5.0 it was Flowmaster Outlaw exhaust. Honestly, I had been turned off to Flowmaster since the 80 series bad reputation on the 4th Gen F-Body cars, but the Outlaws were decently priced so I convinced my wife to give them a try and they gave me the sound I was looking for from the start. That and the wheels and suspension was still on backorder...
I've always done a catback as my first mod. Except on my current car, as I bought it already pretty modded. First thing I bought for it was a reische thermostat and an LFP heat exchanger.
Flowmaster mufflers were the first mod. That eventually grew to headers/full exhaust. Then every single bolt-on you could think of was put on the car. Now it's completely modified with a 331 stroker.
Contemplating a 427 stroker now, or a Coyote swap with a griggs racing suspension. It's an addiction.
And yes I always planned on modifying my car. Ever since I played Gran Turismo on the PS1 in the 1990's before I had a car... I knew my cars were going to be modified.