How do I display the content from one template in another

The button is set as a shared region because it's the same content (a link to the reservation system) used in several places. IE: the top of every page, the footer of every page and listed with each vehicle. Did you view the page? That would make it very clear.

So, if I had 17 vehicles all linking to the reservation system, that would be cumbersome to update each one should the link to the reservation system change( and that is a possibility). Also, if the link changes to an external system ( which it may in the future) That again becomes cumbersome to edit each spot in which the button is used.

Perch custom content won't work because I can't use that on a content template, can I? It doesn't seem to work when I do that. It does work on a master page, but not on the perch/content/templates/fleet-vehicle.html -- which is the template I used to create each vehicle listing on the referenced page.

My template is below. Using perch_content_custom returns 'reservation-link.html' to my live page.

<div class="fleetListing">
        <div class="row">
                <div class="col-sm-8 rborder">
                        <perch:if exists="image_ext">
                        <img src="<perch:content id="image_ext" type="image" label="Add the Exterior Image." bucket="Fleet_Exteriors" help="Add the Exterior image of the vehicle." divider-before="Exterior Image" />" alt="<perch:content id="img_ext_alt" type="text" label="Alt Text" help="Place alt text here to describe your image. It is helpful to use a keyword" />" class="img-repsonsive" />
                        </perch:if>

                        <perch:if exists="image_int">
                        <img src="<perch:content id="image_int" type="image" label="Add the Interior Image." bucket="Fleet_Interiors" help="Add the Interior image of the vehicle." divider-before="Interior Image" />" alt="<perch:content id="img_int_alt" type="text" label="Alt Text" help="Place alt text here to describe your image. It is helpful to use a keyword" />" class="img-repsonsive" />
                         </perch:if>

                </div>
         <div class="col-sm-4">
            <div class="vehicle_title"><perch:content id="vehicleTitle" type="text" label="Vehicle Type" order="1" title="true" /></div>
             <perch:repeater id="AmenityList" label="Add the Vehicle Amenities">
                <perch:before>
                 <ul>
                 </perch:before>
                    <li><perch:content id="Amenities" type="text" label="Vehicle Amenities" /></li>
                <perch:after>
                    </ul>  
                </perch:after>
                </perch:repeater>
             <?php
    perch_content_custom('Reservation', array(
        'template'=>'reservation-link.html',
    ));
?>
            </div>  
    </div><!-- ./nested row -->             
</div><!-- ./fleet listing -->

Thank you, but what do you mean set it as a template attribute? Can you point me in the direction where that is done and let me know how I should approach setting it up that way?

Well, again, this approach assumes I know how to code in PHP, which I don't. Also, where does this code go? My master template? My content template? I'm sorry, but that explanation assumes a lot of knowledge I just don't have. How is it adjusted to fit my circumstance? I really don't understand all that code. So sorry, I don't mean to be difficult, it's just that without programming knowledge the code below is a foreign language for me.

<?php 
    PerchSystem::set_var('lang', 'en');
    perch_content_custom('My region', array(
        'template'=>'template.html'
    ));
?>