Unfortunately Mk2s do produce a fair bit of roadnoise, which is a real killer for bass reproduction.
Another VERY important factor (and often overlooked) is the method of mounting the front speakers. Assuming you have components, it's the mounting of the mid that you need to pay attention to. The key is a SOLID mounting - adding an MDF baffle often helps. The front speaker location on a Mk2 is poor to say the least - A flimsy grill, mounted to a reasonable pod, then screwed to a cardboard doortrim. Not only that, but the pod doesn't even vent to the door cavity - well not until you've attacked it with good sharp stanley knife! The other important thing is to do your best to separate the area in front of the speaker from the area behind.
Some sound deadening would also help - at least directly behind the door speakers (on the outer skin). If the sound from the same system was good in your Mk3, I wouldn't replace your cables just yet!