Thanks so much, Guys, for all the feedback. I was making assumptions about the cord being hard-wired to the heater based on the pictures I could find. But after reading your comments, I called my dealership and they told me I could order just the replacement cord for about $35 (CDN). So I ordered one and I'll let you know if it fits and how the installation went. Thanks StevJ for the heads-up on dropping the cord through FIRST to make connecting it easier. Probably your tip will save me some grief.
UPDATE: I dropped the orange end of the replacement block heater cord down behind the engine then dragged myself under my truck with the expectation that I would just unplug the old cord and plug in the new cord. Turned out that I couldn't even fit my medium-sized female hand into the tiny space far enough that I could get a grip on the old cord to disconnect it from the heating element. I tried every angle. I honestly can't imagine how all the people on the DIY block heater post managed to get the job done unless they all had very dainty hands with 10 inch long fingers. Maybe I'll get some of those portable ramp things so I can drive my truck up on them and thereby gain some working room. If anyone has any suggestions, I'd be happy to hear from you.
UPDATE #2: Took another stab at it today and finally found an angle where I could get 3 fingers sort-of close to where the orange rubber end plugs into the heater element and with the help of a screwdriver, I wedged it out. My hubby then patiently raised and lowered and swivelled the new cord from above the engine until it was at just the right angle then I tapped it with a screwdriver and it went in. It took a lot of tries to get it right though. I can't imagine the mechanics at the dealership fiddleing around like that - there must be some trick to how they do it. Anyway, I hope this post is helpful to anyone else who needs to replace a block heater cord on their 2008 5.7 litre Tundra 4x4. You might want to also check out a thread by "RONE" about how he did his block heater installation - He has very, very good pictures, although they make it look like you have a lot more room to work.