Rewiring standard speakers help!

They are probably crackling because you've used the wrong adapter, you should be using the PC9-404 adapter which runs off the rear RCA's on the aftermarket HU. If you are using the PC9-401 speaker input adapter then this is why you are getting interference.
The easiest way to link up the rears is to just pick up the wiring at the rear amp, which is inside the tupperware sub. However if you wish to continue using the sub then you will need to connect direct to the speakers. The downside to rewiring them and continuing to use the stock sub is that the sub will be barely audible as you will now have twice the power going to the cabin speakers but still the puny 1x50w going to the sub. At least if you carry on with the rears amped you can fade to rear to boost the sub/rears. If you do rewire them then ensure you reconnect the sub to the sub pre-out on the aftermarket HU (if it has one) that way you can whack up the sub level to compensate a little.
The rears are just standard spade terminals aren't they?
They will be different to the ones you get on car audio speaker cables but should be easily available from Maplins or Halfords. To avoid having to butcher the loom you can also buy a standard male/female speaker ISO loom, £2.99 from Maplins. This will allow you to plug in both the stock fronts and the new rear wiring.
http://www.maplin.co.uk/Module.aspx?ModuleNo=99499

They are probably crackling because you've used the wrong adapter, you should be using the PC9-404 adapter which runs off the rear RCA's on the aftermarket HU. If you are using the PC9-401 speaker input adapter then this is why you are getting interference.
The easiest way to link up the rears is to just pick up the wiring at the rear amp, which is inside the tupperware sub. However if you wish to continue using the sub then you will need to connect direct to the speakers. The downside to rewiring them and continuing to use the stock sub is that the sub will be barely audible as you will now have twice the power going to the cabin speakers but still the puny 1x50w going to the sub. At least if you carry on with the rears amped you can fade to rear to boost the sub/rears. If you do rewire them then ensure you reconnect the sub to the sub pre-out on the aftermarket HU (if it has one) that way you can whack up the sub level to compensate a little.
The rears are just standard spade terminals aren't they?
They will be different to the ones you get on car audio speaker cables but should be easily available from Maplins or Halfords. To avoid having to butcher the loom you can also buy a standard male/female speaker ISO loom, £2.99 from Maplins. This will allow you to plug in both the stock fronts and the new rear wiring.
http://www.maplin.co.uk/Module.aspx?ModuleNo=99499

Less work to pick it up from the sub connector, as you don't have to remove any panels.
Here is the pinout for the connector:
Non Bose (17, 18, 20 & 21 are the only ones you need to tap), but you'll need to try them to see which is left & right.
1 yellow – Right Rear Pre-out
2 purple - Screen
3 green – Left Rear Pre-out
9 white – Remote on
17 yellow/red (rear speaker out)
18 yellow/brown (speaker out)
19
20 black/white (speaker out)
21 black/blue (speaker out)
22 brown (earth)
23 red/blue (power)

No you pick up the connection to the speakers from the connection at the sub, you still then have to run this connection back to the HU