I've had really good luck with Simple Green as a cleaner
You can get a gallon of 'Carpet Concentrate' off Amazon for around $20. Oddly, it's not green! They leave the dye out.
dilute it down, spray it on, let it soak. scrub on it a bit, expose the next layer, repeat.
I've used this to clean car parts, as a carpet cleaner, kitchen cleaner, etc. great stuff.