Our 98 jetta vw won't start it just keeps cranking over and backfiring we've changed spark plugs and wires distributor distributor cap

If your VW has more than say 100,000 miles, you may have a valve timing belt that jumped a cog or two. This can make a car run poorly or not at all because the valve train is no longer running in unison with the pistons, spark, fuel, etc. This Jetta came with about six different engines and I am not able to say whether or not yours is an "interference engine". (An "interference engine" will self destruct if the timing belt jumps more cogs or breaks. The pistons will hit the valves if the belt breaks.) I would have it towed to an independent garage who does VW timing belts and get a diagnosis. IF that's what it is, they might do it for $300. Dealer wants $1200. My Dad has a 2001 (interference engine) with only 54K miles. They say to replace them at 60-100K miles. Be safe. I hope this helps.

I forgot to mention that there was a recall on the ignition coil pack on my Dad's 2001 VW Golf with the 2.0 liter four cylinder engine. Not sure if this is the same as your, but I would call the dealer with your VIN and see if your car is included. Have it scan for codes and a jumped belt may show up indirectly as an ignition fault code.