As long as you are only using the after-market connecter and loom to 'trigger' the device you want to install, then the thingummy from Motorworks is exactly the thing you need. You do not need the Denali wotsit.
The loom is fine for powering a sat nag such as a Garmin or TomTom but it depends on how much current your set up is going to draw as to whether the supply will be sufficient to use it as your only feed.
The connector from Motorworks (a bargain at £12, they have been as much as £21 in the recent past
) has three wires, earth, +ve 12v on and off with ignition and +ve 12v on with ignition and off after about a 30 second delay after ignition is off. The last one is great for turning off the ignition, grabbing something from a pannier and getting the ign back on without losing the power to your device.
Of course, if you only need to trigger the device and not have the 12v there permanently, then it makes little difference.