l have a 98 mustang and a 99 explorer but l want o make it so one key works for both. is there some way to do that?

You can have a locksmith re key one of the vehicles to match the other.
If you take the locks out yourself it will save some $$$$.

Have one side cut for the 98, and the other for the 99.

YES it can be done ; you got to take both vic. to a ford dealer and have it done

Yes a locksmith can re-key one or the other to match, or go to the dealer and have them order a set of key tumblers for one or the other and install them.

How much money u got? cause it will cost a few hundred to do it.

Ok bad answer. let me bring u up to speed

On the vehicle, u could cut one side of the key for one car & the one for the other however u have to remember which side for which car.

If not u can code the ignition & door locks for one key code or the other (picking the vehicle u want is key code)

but no matter which way u go, one or both vehicles (one car if u change the ignition lock cylinder on one vehicle, both if u do the half & half key) will have to have the keys programmed to the vehicle as both r PATS system vehicles. meaning it needs the right code to start the vehicle programmed to it

so a few hundred for sure

l will rather fallow old man dirt suggestion,,but you still have to program your key to match the other cars p.a.t.s. system