Interchangable, the ones with the wire are easier to put in. Carry an extra one with you. I had my first one fail this summer in my Polara in my avatar. I was quite a ways from home and got flat bedded back home The bad thing was, I had a new one in the glove box. Mine started missing and backfiring thru the exhaust, I could drive slow but once I sped up the backfiring got so bad, that I thought I was going to blow my mufflers off. I had never had one go bad before.
