03 toyota highlander

W2W ( hardwire ) mode always works.   For that vehicle it's only a transponder bypass function, so we are only talking about 3 wires ( +12V, ground and GWR ) that have to be run ( instead of the D2D cable ).  Some bypass modules include the Keysense wire, other use the R/S's GWR to handle it.

For a better answer we would need to know the R/S make & model and the bypass module used.

As long as you can make solid connections, hardwiring is always the best method. How many times do you want to go back under your dash? I personally would rather take an extra 15 minutes during the install knowing that I won't be going back in there, and if I do it will be MUCH easier to diagnose a hardwired install than a data install. How do you test what the bypass and alarm are saying to each other if you can't see the communication in simple 12v/ground signals?

On top of that idatalink/flashlogic always tells you when you call for tech support, if you are using D2D to switch to hardwired method before they can help you.